I don't see it varying a lot with class so much as with race. Paladins don't have to be uptight (nothin' in the code says they have to be celebate), and could actually be quite permiscuous, as long as they treat their paramours with the same dignity, respect, and saving grace that they treat everyone else with. Falling in love with a paladin may be (in the paladin's eyes) the best thing that has ever happened to the street urchin woman or the prostitute, because the paladin has the chance to redeem them...
....hm...but society would condemn them both....I think I've got a story idea!
Clerics, too. Nothings says that they can't have families or have intercourse.
Odly enough, I would see Monks as probably the most restrained, since they're the most likely to deprive themselves of *any* worldly pleasure, including this.
As far as races go:
Humans: Without the advantage of the Western culture's domination by the Catholic Church, there's really no telling what this may become. I just sort of go with today's ideas for the most part, because it's easier to deal with.
Dwarves: More conservative. Dwarves generally don't cotton to any sort of promiscuity or messin' about...that generally goes with patrilineal clans, anyway. I do see dwarves as fairly strongly patriarchal, though IMHO, women in dwarven communities are seen as special treasures, very close to the forces of life and death, and the most mystic of the dwarven people. Dwarven warriors and kings are male. Dwarven priests and magic-users are female. The patriarchy is enforced with a taboo against using wizardly magic, which, to dwarves, is a dangerous and potent force that mere mortals shouldn't mess around with. There could easily be legends and such detailing the dangers of women who went and got magic and then helped bring down a dwarven fortress or something. IMHO, to dwarves, Magic = Women = Spiritual, and that's subordinate to Might = Men = Physical, but still a very valuable peice of reality.
Elves: Well, IMHO and IMC, elves have very low reproductive rates...because even male elves have a sort of menstraul cycle. This means that every 5 years, when the stars are right and the moon is full, elves become potent for a week, and gather into great elfmeets to reproduce. Basically, big elf-orgy. Other than that, they can't reproduce...which means that contraceptives are NOT a problem. The problem comes when the stars are right, and they're nowhere near an elfmeet -- that's how half-elves are born. They get the urge, and often satisfy it on the nearest creature. Elves have a strong sense of artistry and enjoyment of the physical world they live in, making them ready and willing to include intercourse as part of that experience. Elves also have very little dichotomy between male and female -- for a large part, to elves, they're one in the same. Thus, many elves may be what would be termed bisexual, but, to an elf, beauty's beauty, and it doesn't matter what gender it is. This leads to a certain custom IMC of calling humans and others with sexual tastes that aren't the norm to be labeled "elfish."
Gnomes: Somewhere between the elvish thirst for pleasure and the dwarvish rejection of it lay the gnomes. Though not particularly concerned with keeping strict monogamy or with having intercourse with anything that seems like it might be fun, gnomes have a sense that it is important to keep family unity, and that it is enjoyable, so there. I think gnomes as more structured around the family unit than most other races. When a gnome has a family, it is their responsibility to get another parent, and to raise them according to gnomish custom. Of course, after the kids are all grown up, the gnomes are free to go their own ways, without hard feelings. The only bitterness that erupts is when someone skimps while the kids still need a stable family. Then, entire communities of gnomes will gather to oust the charlatan. But, if you don't have a family, you're free to wander and do as you please. I think that some form of contraceptive is a gnome-bachelor's best friend.
Halflings: I could see halflings as basically polyandrous, perhaps similar to animals like elephants, where the family line is traced through the mother, and males exist on the outskirts, allowed in occasionally to reproduce, and then moved on when they reach of age or are finished raising the kid. The cycle probably repeats itself every few years, with men perhaps returning to the same women (or going to different women) year after year. There are probably male competitions to impress the ladies, in order to get chosen. All the husbands of a particular woman help raise the kids she gets, but she also has to do it, so fewer husbands = fewer kids = fewer funds drained. But I think halflings like big families, personally.

I mean, when your major deity is a variation on the mother-goddess, how can you *not* have a strong population of women?
Orcs: Orcs are sort of the reverse of elves, IMHO and IMC, in that they are basically bisexual, but see it in a very different way. To an orc, intercourse = power, and so the more people who want it from you, the more power you have. Show off, show your might, triumph over the enemy, and be worthy of the idolization of others in the community. However, be careful, because if you pick someone who isn't actually impressed by you, not only will you be ridiculed (reducing your impressiveness), but they may choose *you* the next time around, to let you know just how weak you really are. If you are the choser, you have power over those who whish to be chosen, and the reason they want to be chosen is because they recognize your power. Orcs often participate in what other races may call a rape, in that they'll sometimes simply take the mate they want. However, in orcish society, this is, again, a sign of dominance over the mate...if they didn't want to be taken, they should be able to defend themselves. This creates an especially bad environment for women, because they have to either rely on themselves or on men they dominate to care for their young, and it's never safe to trust the men you dominate -- they may try to dominate your young. Fortuneately, there is a bit of a prohibition against this -- if you're a dominated man trying to take advantage of you're domineer's kid, you better bet that the domineer is going to come down on you like a ton of bricks.