That has to do with her attraction though, not yours. You would be being respectful of her preference (good), but your attraction to her simply is a reality, just as her intrinsic inability to be attracted to you is also a reality. Likewise, if a gay woman sleeps with a trans woman because she's attracted to all her feminine qualities, it doesn't make her any less gay.
The discussion is not offensive to me, I should clarify that when I talk about someone's beliefs being transphobic I'm not passing judgement on them as people, they aren't bad human beings, from my point of view it's simply a prejudice that's absorbed culturally. People don't seriously examine it until they are put into a situation where they have to actually make a intellectual choice. A lot of people, I would say the majority of us, have these kinds of latent biases that we never really bother to pick apart and instead just act on in a kneejerk way.
A person's sexuality is distinct from their capacity to be attracted to trans people, it doesn't make sense to state it as a preference in and of itself.
I'm going to unpack a thing here, bear with me, here is why I see the two things as distinct; If you met someone and were very attracted to them, you got along great, even went so far as starting a romantic/sexual relationship. Maybe you're even together for years. One day they confide to you that they are fully transitioned -- your response to that is not actually going to be about your attraction to trans people. Your attraction to them was already concretely established, it's no longer a question.
People might make arguments about "Well I wouldn't want someone to keep something like that from me" or something along those lines, but that still doesn't have to do with the person being trans or not -- it has to do with value judgements concerning honesty, trust and openness etc. Which are characteristics independent of gender identity.
Or, if it helps to think of it another way; if you date someone who has not transitioned, and then they come out to you as trans, you cannot then retrospectively claim to have not been attracted to them simply because that fact came to light. You might lose attraction to them going forward from that point because their body may change to become more feminine/masculine, but it's not because of their having a status as being trans. Just like your attraction to a gay woman doesn't change based on the fact that they're gay.
These are scenarios that could potentially happen with a trans person, but it would not happen between two cis people. A completely straight cis man is never going to inadvertantly fall in love with another cis man and have a relationship with him without understanding it as being a bi/gay relationship, this is the difference.
The blanket belief that one simply cannot be attracted to trans people is misleading. I would go further to say that belief is a roundabout way of obligating trans people to live not as their target gender, but to forever live as an "Other". It's a conception of transness that functions on stereotypes and requires them to perpetually qualify their relationships to other people for their entire lives based on that medical history, regardless of whether or not those people percieve that they are trans or not.
It's a way of saying "It doesn't matter if you pass perfectly, or even if I find you attractive, you're still required to out yourself and can never actually let go of that old identity." Which, to me, seems unfair and is sort of an insidious manifestation of the trope that trans people are being dishonest and deceptive, that they "fool" people into being attracted to them rather than people simply being attracted to them naturally.