i'm not sure how i would react, to be honest. i think in general, i'm fairly open-minded about gender, and I don't align myself with the gender critical position around trans identity. but that is a separate question to whether i would want or be able to continue a relationship with some who transitioned.
the reason I hesitate to say 'no, i wouldn't stay with them', is because my current partner has been curious about his own gender since I met him, and has started to experiment with make up and feminine clothing. not dressing 'as a woman' per se, but more just experimenting with gender presentation. he's happy using male pronouns and being identified as male, but if pushed to choose, I think increasingly he would identify as non-binary or genderqueer.
I've been surprised, in many ways, but how un-bothered I am by it - and indeed, the reverse is true. i've enjoyed watching him grow and feel more comfortable in his own skin, and if anything I'm just more and more attracted to him. like another poster, i'm bisexual but i'm not sure that's really what's going on (particularly given the women I'm generally attracted to is a pretty different vibe...).
if he chose to fully transition - which i think is unlikely, it's not where his mindset is - I don't know how I would react. but I figure we're on an interesting journey together, and I'm not going to make assumptions about how i would feel, because I don't think I could have predicted where I am now, either.