Assuming we are treating Mino's gender change as something with parallels to HRT in terms of body and mind changes, expansion of sexuality is something that can and has been shown to happen to a number of people under HRT, although not necessarily a case where one person went from purely straight to purely gay (at least, not in the sense of, say, liking only girls as a man to liking only girls as a girl), rather, a person going from a purely straight/gay standpoint to a more fluid one, even if they retain a preference.
In my opinion, Mino was never 100% straight necessarily, but is not a 100% lesbian either necessarily (at least with the current information given, it is perfectly possible for it to go that way as well), they are just experiencing an expansion of their sexuality as new barriers are broken and pushed in directions that they weren't aware of. The most often given explanation to sexuality exploration post-HRT is due to said barriers being broken, and seeing things from different points of view because sexuality is not a binary on/off switch, but something that fluctuates heavily.