But Elves aren't Humans, so why would they share the same sexual make-up as us? We've got 2 main groups, Heterosexual and Homosexual, with smaller groups of people being Bisexual, Asexual and any other variation on the theme of Human sexuality that I've missed. There are contentions about which of these is the 'correct' one, but that is more to do with our outlook on what constitutes a gender, and what is 'correct' for that gender to desire sexually.
Elves' sexuality is going to be conditioned by their extended lives. Once you strip away (sorry!) the need to focus on getting someone you want and having kids with them being one of the prime movers of society, might you not then become more about what's in a prospective partner's head, rather than in their pants? Certainly, there will be those who prefer those of a particular gender; but absent attitudes that say that any particular gender-pairing is wrong, moving outside of that established preference will have no repercussions.
If Human society moved past the differences in the genders mediating what is appropriate in terms of what is right and wrong, then I reckon it wouldn't be long before Humans are like the Elves. The longer lifespan, though, would allow for more time being spent on establishing a relationship on levels that might appear smothering to us, now, but with the benefit of time and luxury of choice, might become natural.
That didn't come out the way I was thinking it. It sort of gets across what I'm wanting to get at, but is a lot more clumsy. Hey ho.