if you were together before you transitioned or whatever, i think it's different. however, if a trans man is still in womens spaces and actively seeks relationships with lesbians, i think that is where the problem lies. why would a trans man be in womens spaces (with the exception of safety concerns, obviously)? why would a trans man actively seek out a lesbian to date, invalidating his identity and even hers? it goes against the whole purpose of being seen as a man. i get WHY people would but i don't agree with it.
i wouldn't date a lesbian because it would make me feel really dysphoric. i am a straight man and i wanna be with a woman who likes men (whether she is bi, straight, whatever, as long as she likes men and would see me as a man). i spent too long wanting to be seen as a man and accepted as a man to deal with people thinking i'm just some lesbian. them seeing me as a lesbian is probably unlikely because i have facial hair, etc. but because of that, i also don't want people to think my lesbian partner is with a man if she is committed to her lesbian label and a proud lesbian. i think that can cause friction and make her feel as if her identity isn't being validated which causes issues in the relationship (i feel this can also happen with straight women who date trans men too). people in the LGBT community usually take awhile to accept who they are, and yes everyone else including the own community tries to invalidate them for going against the "norm" like a lesbian dating a trans man. "oh, you sure you're a man? are you sure you're a lesbian?" so idk.