Being already married and stuff aside, as a straight cisgender male I would honestly have a hard time with it. Like, my parents' neighbor is a trans woman and I'd be lying if I denied noticing her (covered) boobs when she was hanging out in the back yard. But if I'm going to have a romantic relationship with someone, there are physical traits that attract me and others that don't... And I would need to be attracted to my partner, both with what's typically visible in public and what isn't.

It's nothing personal, and I truly hope their new body (whatever that may be) works for them and they find the love they want. It's just not what I'm after.

To use the cliche as a tl;dr, it's not you, it's me.