Some percentage of humanity has always been homosexual. Some has always been heterosexual. Most though have always been something somewhere in the middle. If you think of sexuality as a bell curve, heterosexual on one side, homosexual on the other, most are somewhere in-between the two. In this way then, heterosexuals denying the vast majority of people full legal equality is an Aparteid-like phenomenae. Heterosexual Apartheid if you will.
Because this variation has been with us all along though, like the line in "X-Men: Days of Future Past" "if there are homosexuals among us, and have been all along, they're living alongside us just fine." Indeed. Only thing that's changing is wider social acceptance of reality. It's just as acceptable to be homosexual as it is to be heterosexual because neither's the majority.
How many heterosexuals have had a gay sexual experience or fantasy? How many experiences of fantasies does it take before you're not considered heterosexual any longer? This type of "heterosexual" defines most heterosexuals. Homosexual acts and ideas aren't unknown so why identify as heterosexual if you're enjoying some degree of homosexuality? Until very recently you couldn't and had to keep that all to yourself. Times are changing though. And a broader acceptance of this reality is becomming the standard.