From my understanding of Tolkiens world (which is hazy at best, please correct me if im wrong) the process of Elves conceiving a child takes years and is a very delicate process, the fetus' die can easily when compared to humans.

In order for Elves to keep their population numbers even at a consistent level would be difficult to maintain. Id imagine on a cultural level Elves would feel its their responsibility to have heterosexual relationships, regardless of their preferences, to perpetuate their race.