The problem with questions like this are that 1. they invite the reader to project themselves into some specific scenario and answer based on that; and 2. those scenarios usually implicitly assume some kind of misbehaviour on the part of the trans woman.
For point 1.:
For instance, you suggested, "What if you are about to have a one night stand and you discover she has a penis"? Well, it wouldn't be transphobic to refuse to go ahead that evening, because maybe you aren't attracted to penises and that's a perfectly reasonable preference. The problem isn't that she's trans (plenty of trans women don't have penises), the problem is that you find a penis to be a deal-breaking characteristic.
You then suggested "What if it's your wedding day?" For most people, sexual compatibility has already been established - no surprise penises here - but there are a two reasons this might be a deal-breaker for you. (a) Maybe you feel upset that your fiancée didn't trust you with this part of her life story, and that makes you doubt the relationship. This would be a sad story, but not transphobic. (b) Maybe you are so disgusted at transness, intrinsically, that you can't go on with the relationship. This obviously is transphobic.
So the answer boils down to "if you break up with a trans woman because you're transphobic, you're transphobic, but if it's not because you're transphobic, you aren't transphobic," which is a tautology and makes the discussion stupid and meaningless.
For point 2.:
Discussions like this tie into a stream of rhetoric which is actually dangerous and has real life consequences for trans women. It propagates the idea that you would just "find out" that you've been "trapped" into having sex under false pretences; that trans women are essentially out to rape you. In fact the opposite is true, the advice in the trans community is always to disclose as early as possible. You want transphobes to know you are trans straight away so they can just type an angry rant and block you, because if you meet up with them then tell them you might get beaten up or killed.
In extremis, the
"trans panic" defence - based on the idea that trans women are trying to "trap" or "trick" men into having sex with them - has been used to literally justify the murder of trans women.
This isn't a conversation worth having, it's dumb schoolyard gooseberry fool based on regressive talking points.