Before I accepted that I'm trans, I also considered myself a straight male, and when I saw a penis I would turn away as if I had just witnessed a gory crime scene. Almost one year into HRT, though, I view it as part of a whole person and what I think about it depends mostly on my relationship with that person.
Much of this is HRT turning my sexuality more inter-relational, but of course that can't be all of it, as a lesbian would be less open to penises than I've become. So, my theory is that there has always been a latent or suppressed part of me that has been more open to penises than I've realized.
This is one of my theories about what's going on with you (except from a cis male perspective). My other theory is that I know that some straight guys can re-frame a pre-op mtf's penis as a female penis or large clitoris. So, either you like her despite her penis, are neutral towards her penis, or like her because of her penis. All of which are cool. Just keep soul searching, until you figure it out, so both you and your partner know what to expect.
As far as what you should be doing with the woman you're seeing now, it seems like you're already doing great. Just make sure she's fine with you contacting with her genitals, as that can be very distressing for some trans women.
