Attraction isn’t really exclusively based on sex, though. It’s about a whole range of factors, including how someone presents their personality and their physical traits. Many trans women have a lot of traits people find attractive in women generally, so being attracted to a trans woman is consistent with heterosexual attraction.

This means that liking trans women doesn’t automatically mean you’re attracted to men, so calling it gay is an oversimplification of how attraction works.

You could argue that in a strictly biological sense it is homosexual, but that ignores a whole range of social factors, which isn’t really how the term “gay” is colloquially used.