Sexual identity is a sliding scale. Everyone has some small slight inclination of gay in them. Everyone has some small slight inclination of straight in them. The proportions vary depending on the person and how willing they are to acknowledge their genetics. Some people produce testosterone in greater quantities than estrogen, but both are produced. The link between those 2 and sexual identity has been strongly supported by science.
I point to Roman culture as the example. The Roman aristocracy believed that sex with women was for procreation and recreation. True love could only be found between men.
Some people lean so far one way or the other that they find the opposite absolutely unthinkable.
Lesbianism is put on a pedastal in our culture for young women, so it's not surprising that some people that aren't naturally more inclined to it would indulge in it by choice. Others are absolutely 100% opposed to heterosexual intercourse. Some so much so that they voluntarily have surgery to become the opposite sex.
So in summation, some do choose it, but I'd highly doubt that "most" do. Gay men are one of the most discriminated against in our society. I can't see someone choosing that for themselves.