No mate, actually, it's not. You could say it's level 0 of bisexuality instead.
I could list several pretty sane reasons why topping a beautiful TS could be emotionally and sexually pleasant (because it's a closeted topic which makes it kinky and sexy, because of extra levels of feminine behaviour in a TS, because of nice eventho' fake boobs, because of better body than 80% of women that are pretty much untouchable for me just because they are f-ugly and - which is i fact quite gay - because you actually top an XY person which creates that kinky confusion in your brain (and is quite empowering - yes this is quite a gay feeling imo)
So it's rather kinky, definitely not heterosexual behaviour yet not a gay behaviour really as well and studies on this topic provide strong evidence that GAMP men - i.e. men that are into transsexuals - have sexual arousal patterns very similar to pure or almost pure heterosexual men (btw. heterosexual men do have gay sex sometimes - it happens when they are at the exploring stage of their developing seual identity which matures around age 27 or so). Moreover, GAMP's are not aroused by gays/men attributes only by feminine aspects of transsexual person.
Study on GAMPs
So they are about 8/10 on Kinsey's scale where purely heterosexual person is 10.