Canadian Politics Today

Validation.

If a straight woman wants to have sex with a trans-woman, it means she actually sees him as a man, which isn't how she wants to be seen.  The relationship would be based on, from the trans-woman's perspective, her being misgendered.  At best the straight woman is into the trans-woman because she doesn't understand the trans-woman's identity... or worse perhaps she has a fetishized view of them.

If a lesbian wants to have sex with a trans-woman, it would validate the trans-woman's identity as a woman. She would have a lover who sees her as she sees herself-- as a woman, not as a cross-dressing man.

What's going on at campus LGBT clubs and other gay spaces right now is that there is a lot of pressure to be trans-positive, because if people decide you're not trans-positive, you're not allowed.    So you get conversations that go like this:

Gaia: "Hi Becky, I was wondering if you'd like to go out with me Friday."
Becky: "Oh hi, Gaia... I'm sorry, but I'm gay... I'm not into guys."
Gaia: "Well I'm not a guy. I'm a woman. I'm still transitioning, but I'm a woman. So if you're into women, there should be no problem!"
Becky: "Uh, I'm sorry, I'm just not really attracted to you."
Gaia: "why? Is it because I'm trans?"
Becky: "No, it's just that--"
Gaia: "It's because I'm trans, isn't it! Oh my god, Becky is transphobic!  Becky's a TERF! Hey, everybody! Becky's a TERF!"
Everybody: "OMG Becky how could you" "Becky WTF" "GTFO, Becky, we don't want you here." "Get lost TERF scum!"
Becky: "but guys, I just--"
"Becky, you're not welcome here anymore. Just leave."

and of course after the other girls see what happens to Becky, you get this:

Gaia: "Hi Katie, would you like to go out with me Friday?"
Katie: "uhhhhh  ... yes?"

Because being perceived as being transphobic means being disowned, everybody is desperate to prove they're trans-inclusive.  Whether it's by going on a date with Gaia, or by being part of the lynch mob that strings up Becky, people want to prove they're trans-positive so that they don't end up like Becky.

Cybercoma will probably tell you that this is just propaganda, that this doesn't really happen. I have read enough stories from women who have been on the receiving end of this kind of witch-hunt to know that it does happen.
-k

In your scenario, Becky calling a Trans woman a 'guy' while turning them down was disrespectful, unnecessary, insulting and likely to make things go sideways. Was it transphobic? Well ... ?
It's never wise to invite argument by trying to explain why you're not attracted to someone. Saying "no" already says that you're not.

I'd wonder how widespread a problem it is that a peer group that would join in and try to bully a person into dating someone they already turned down, 'shun' them the way you describe. Really?

Everybody has to stand up for their own right to choose their own partners for their own reasons,  like Katie and  'the girl' in your previous scenario will have to do.