Elves as a whole are not gay, that is a silly question though it is quite a headline to grab attention. But both the assumption of longer lives = More openness to things like sexual experimentation and the comparison to classical paganism is ludicrous for different reasons. Generally speaking the older the person is the more set in their ways they are, and there is no reason why elves would be different, if someone was straight for the first 40 years of their long lifespan, there's no real reason to suspect they would be open to homoerotic experiences for the next hundred or so years. While simple law of averages would make you think it would, people, and by extension elves, don't work that way. And while it is obvious to see classical Greek and Roman, (as well as Celtic and Japanese) influences in the High Elves' design, their culture is entirely unique and likely doesnt have the hangups of the Greek sexual liberalism, (and thankfully, a lack of pederasty), as Gav Thorpe mentioned the high medieval influences in their society likely point to this. Also a better example would be their view by other races. If elves were gay as a culture, the Dwarves CERTAINLY would have said so given their incredibly masculine and conservative culture and the influences of such a gay culture would have reflected strongly in the Human Empire at the least.
So, no, I don't think the Elves are majority gay or bisexual, even if they almost definitely have gay and bisexual members of their society.
Now the Drucii on the other hand almost certainly would be majority bisexual, but not for any virtues associated with the sexuality, but rather because I can simply imagine Dark Elves not caring what territory they plant their standard.