Speaking personally, I wouldn't think of it as having a relationship with a transman. As far as transmen and transwomen are concerned, I tend to disregard the 'trans' prefix. Women are women and men are men. The bits and pieces make no difference, at least as far as I'm concerned.
Malachite does touch on something that, for me, would matter more than anything physical. A degree of empathy and that bond which comes from shared experience, that is a definite advantage in any relationship.
Speaking hypothetically, I would happily enter into a relationship with someone of any gender, as long as that spark was there, and they possessed the qualities which I personally find attractive. For me that isn't physical.
Everyone's different, hon. And what one, or even several may feel about something... that doesn't make it the rule.