*plinks down a handful of cents* (this way exceeding the basic two.)
I'm seeing a lot of ideas being confused here. The differences between societal alignment vs. personal alignment, between alignment vs. personality, and at times even reality vs. fiction seem to be missed quite often, so here's my attempt to sweep away some of the debris.
First and foremost, the "pure" Lawful Good take on relationships/sex/love would be to seek stable, long-term relationships. The Good vs. Evil portion of alignment has minimal impact on this, but Lawfuls prefer stability, predictability, and "comfort factor" while Chaotics prefer intense whirlwinds. Anyone trying to confuse these facts just wants to earn a "my way is the right way" stamp of approval, which strikes me as rather silly and juvenile.
That doesn't quite close off the possibility of the promiscious paladin, though, any more than it does the possibility of a paladin who enjoys gambling as a hobby. Enjoying gambling is fairly obviously a chaotic trait, but I think we'd all rightly be upset at a DM who stripped my character's paladinhood for that. So long as it's kept under control, it's a quirk, not enough to make my character unreliable enough for proper Chaos modifiers. Similarly, promiscuity in many campaigns is only mildly Chaotic, not enough to send the paladin off his stride. (All this does depend on the paladin's intelligence, wisdom, Sense Motive score, experience, and the way the DM handles human nature, mind you. While I'd show said paladin how other people are setting themselves for hurt and he's offering himself as an accessory, many gamers see humans/demihumans/humanoids/etc as rational actors and sex as a morally uplifting sacrament. In games like those, if there's no harm in orgies, why not?)
This all happens regardless of culture. Lawfulness means "likes predictability and order", not "follows the local customs mindlessly". (Lawful characters will tend to follow local customs, but that's more out of a desire to not make waves than anything else.) Real-life examples of Lawful cultures need to keep in mind that even these cultures had a large number of non-Lawful members. Those Chaotics and Neutrals would likely drive a market for vices, but that only says that Chaotic tendencies need to be vented, not that these activities are actively smiled on by Lawful individuals.
So that clears up many of the Lawfulness issues there. The Good/Evil axis is less of an issue here. Good characters will want to keep their partner happy (and prevent their partner pain) while Evil ones will not care. This can apply to either end of the Law/Chaos divide; the Lawful Good attitude will be towards a harmonious marriage, the Chaotic Good attitude will be towards open but nontraditional romances (or a series of open, communicative, but short-lived flings), the Lawful Evil attitude will keep one or more partners as virtual sex slaves (concubinage or a "the wife's place is in the kitchen!" marriage), while Chaotic Evils would keep actual sex slaves and indulge in kidnapping/rape to get their jollies when they have the opportunities. Keep in mind the above caveat about atypical attitudes in characters, though.
And finally, stop trying to define Neutrality as if it were an active force in either alignment issue. It may be in some cosmologies, but there has to be room for the people who just don't take an active focus in alignment issues. The "Pure Neutral" attitude would be to just sit there and do nothing, but I for one dislike the earlier attitude that Neutrality is the razor edge between the alignments. (Sorry for the rant, that last one's just one of my pet peeves.)
