Like, even putting Erica aside (which absolutely shouldn't be, but putting that whole discussion aside regardless) the spoilers regarding Rin sound 100%, unambiguously transphobic regardless. Like, regardless of if Rin can actually be considered transgender, the whole Rin turning out to have a penis and Vincent being shocked 100% embodies transgender "trap" stereotypes, that transgender individuals are trying to hide what they "really" are and trick people into having sex with them unaware, and that's exactly the kind of stuff that gets transgender individuals killed and leads to them killing suicide each and every day, those kind of stereotypes and that kind of imagery. No one who's transgender actually does that stuff, tries to trick people like that. They're more afraid of that possibility than anyone, that someone might feel deceived in any way and fear for what would happen to them in that kind of situation. It's just not a thing.
And that's why it's not just Vincent's reaction to that scene that's a problem: it's the fact that a scene like that is even in the game, because that's not a thing that actually happens, and it only ever comes up in transphobic nonsense and anti-trans propaganda and argument in the first place. So it's not just Vincent reacting that way that's a problem, the fact that there's even a scene like that in the first place is a huge issue and is the embodiment of anti-transgender gibberish.
And for the game to use that kind of imagery for Rin, regardless of if that character can be considered transgender or not, it's clearly invoking the imagery of the "trap" and that's not cool at all, 'cause "traps" aren't real and no one would do that, and just perpetuating those kind of images and stereotypes, that that's a thing, can only hurt transgender individuals and even putting Erica aside, to have a moment like that with Rin is completely unacceptable and makes everything that much worse that the game has these issues in regards to both characters. Especially with the twist of Rin apparently being an alien or whatever, how in any way it could be remotely justified for the character to invoke the image of "the trap" to begin with as that would have so, so, so easily been prevented by just having the character explain what's going on to Vincent to begin with the way actual people do. Instead, having Vincent freak out like he apparently does and in fact be surprised by that "revelation", what's the point of all that if not to be transphobic as fuck? Everything about Rin sounds terrible and over the line, even putting this whole discussion of Erica aside.