Sex wierds some players out.
Whatever happens in the world, the main thing is to have a good grasp on your group.
Fer'instance, for me, with my Cultural Anthro background, I like to see sexuality as I can see it in real human societies. I mean, homosexuality, bisexuality, or even a kind of asexuality are nothing new, and certainly not a human-only thing. Every culture has a way to deal with it, often surrounded by a lot of ritual, and that's the same IMC. Usually, I have the reactions to it vary based on race. I've also got a Religion background, which gives me way too much knowledge on some of the more sexually explicit myths that mankind has created...
In general, it's assumed that it's been around and different cultures have dealt with it in different ways, depending upon what they need/use/see sex itself as being for. Which is pretty much how modern society's views (fractuous though they are) came to be.
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Fer'instance, Dwarves are traditionalists who enjoy law and order. They deal with gay dwarves and lesbian dwarves as they would anything else -- find a place, put them there, and live alongside them. Dwarves are also quite reserved, so even the more "open" dwarves are rather reticent about what, exactly, they enjoy sexually, and would never discuss it except with a potential partner. "Partnerships" are as much about functionality as rommance, and functionality includes raising a good dwarven family or helping dwarven society as a whole. The ones unable or unwiling to make the babies (including the castrated, the adventurers, and the same-sex-inclined) contribute in ways that those with families cannot or should not, such as by being soldiers in the army or traders to foriegn lands or the like.
Most Fey are fairly hyper-sexual, and elves are an extention of that. Try anything and everything, enjoy every aspect of it. Currently, there's an Unseelie Fey character IMC who was cursed after having sex with Loki, and the characters make in-character jokes about how he's likely to have sex with anything. Elves enjoy art and beauty, and lovemaking is included in that (though IMC, elves are also rarely fertile and come together in an Elfmeet to have a massive reproductive orgy once every decade). Because elves are open (sometimes to the point of annoying th emore reserved they associate with), being eager for sex, especially with the same gender, is often seen as being "elvish."
Gnomes are traditionalists in that they enjoy a well-functioning family, but they're also remarkably inventive and sensual critters. "Gnomish whores behind closed doors" is a common phrase for "Anything could happen." Gnomes probably experiment a lot growing up, only to find their favorite and settle into changing it up as they go. Families aren't a requirement like they are in the dwarven society, and same-gender relations are generally copacetic.
Halflings have something of a communal lifestyle, formed under a matriarchy (as befits a race whose main creator is a warrior-goddess paladin). The mother leads the caravan, and many women have more than one "suitor," though marriage is uncommon. Manly kinship is rare because of the matriarchal focus of the society, and many are mistrusted because they don't fit in. Womenly kinship is more common and generally accepted.
Orcs are more instinctive and bestial. Like the cruelest, most instinctive beasts, they *take* what they want, without any concept of rommance, love, or compassion. They are monogamous, because they must defend their mates against all comers, and often separate into an alpha male/alpha female pair, and the many that contest around them. Same-sex pairings are uncommon simply because the chemicals rarely flow that way and their society doesn't really know how to handle it. Often, they simply become outcasts, unwilling to fight for a mate they have no interest in, but fighting and subduing those who often have no interest in them. Still, there's no compassion about it -- they hunt down and wear away at whatever they desire.
Tieflings, touched by evil, enjoy suffering. Sexuality for them is a constant power-game, they like to dominate or be dominated. It's not simple bestial sex, it's an elegant manipulation of other's emotions for their own enjoyment. Same-sex unions are no exception: if you can do something for me, I can do something to control you.
Aasimar play hard to get to test those who would court them. They want to see the best out of everyone, and never settle for less than they (think they) deserve. Many are abstinate because of their high opinion of themselves. When they love, they do so deeply and truly, without pretention, and with an almost embarassingly slavish devotion. They aren't for the casual encounter -- only the True Love will satisfy them. They want something as spiritual, not something physical, though they probably achieve as much satisfaction from a noble devotion as most derive from a healthy sex life. Same-sex pairings work just like opposite-sex pairings since reproduction usually isn't an issue, devotion is.
Zenythri are all about the perfection of the act. They're not about to experiment with a variety, but they will hone what they do to a connsumate skill, effective for everything they want to achieve (be it reproduction, enjoyment, manipulation, whatnot). They may not have much variety, but what they do, they do VERY well. Some will hone same-sex engagements as much as they would hone anything else, and would likely do it exclusively.
Chaonds change it up, and love a variety of experiences. They don't like one partner for long, same-sex or not. They're likely to experiment as much or more than the gnomes, but don't really stop. They don't often marry or even date, instead delighting in the random encounter.
Bariaur have some deep gender divides born of biology, and have well-defined roles based on their gender. Those who don't fit into the typical male-and-a-handful-of-mates roles do as the dwarves do and find different roles, if they don't just leave and form their own little communes. Entirely acceptable -- some bariaur spend their entire lives just wandering around itenerant, or forming their tribes far away from their brith tribe.
Mephits are genderless, so it's not an issue at all for them. They reproduce with a spell, though they don't even really feel a need to do such a thing. If they engage in any kind of sex act, it'll be kind of unsatisfying, because they've no real reaciton to that over anything else. Many probably wonder what all the hubub is about.
Githzerai, obsessed with personal choice and acceptability, have no official problem with same-sex relations, but any sexual relation is something of a problem for many of them, tempting them away from the ascetic lifestyle they so enjoy. Young ones are always needed, but more from a functional standpoint than a romantic one. They really rob the act of it's rommance, reducing it to a functionality only. You reproduce, you move on, and that's the only cause for it. Sex for pleasure doesn't make sense to them (most pleasures don't). If there are any openly gay Githzerai, they are doubly odd, both for being interested in sex and for being interested in non-reproductive sex at all. Ousting them from the monestaries of Limbo is about as bad as the Githzerai are willing to do to their own, and the homosexual fall under this curtain as well.
Githyanki, more appreciative of the pleasures of the flesh and mind, definately see the motive behind it, and enjoy it very much. However, it often falls to becoming about their own personal power. They may very well sleep their way to the top, whatever acts may be demanded of them by whatever gender is in power. Gender doesn't dicatate power in Githyanki society, so whoever has the power becomes the target of the sexual advances of most other Githyanki, male or female.
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I don't think in-character jokes or biases are unacceptable -- indeed, I think my suspension of disbelief would be strained if it was "just as normal as everything else!" unless the overall culture was absurdly blind to the difference. Sexuality is prominent in myth, in legend, and in fairy tales (though it is conspicuously absent from most modern takes on fantasy). It's a big part of the fantasy world of humanity, it's only natural that it be considered in games based on that fantasy world.
I've had gay PC's. I've had sex-obsessed gay PC's, horribly cliche gay PC's, and PC's who just happen to be gay. I've had "80% female, 20% male" PC's. Lesbian PC's (who were so reserved it never really came up). PC's who take vows of celibacy. Genderless PC's.
It's a big world out there, full of all sorts of critters and all sorts of cultures. To ignore the difference is almost as insulting as trying to eradicate it, IMHO.
And Obryn does have a point. If it's not an issue, it doesn't need to be constantly pointed out. If you're constantly pointing it out, you as a DM are acting out of character. Mention it, let the players know it's there, but if it's not a thing, it's not a thing. All-Lesbian organizations would strike me as VERY out-of-character for a setting where sexual orientation doesn't matter. Because if it doesn't matter, why would that be a defining characteristic of the organization? Why would it be all-lesbian if a lesbian isn't anything of note?
. But I don't expect that by doing that, my players will feel anymore immersed in my campaign than they do when I give them a good room description. 
