If you are imagining the other gender during the act, I think definitely not. Your hand might act as a stand-in, but ultimately during masturbation, that's all it is. For this reason, Toga's sex with Izuku!Toga could not possibly be gay, because though Toga is still not technically a real boy, she is very clearly being a stand-in for one. This beg the question having sex with a transgender is being gay? Both person are female and thus it should be.