I had relationships with women during my adult life, up until last year, because pre-transition, although I was attracted to men, gay men did NOT attract me, and I couldn't figure out how to be with a straight guy because, well, I looked like a guy. I was intensely lonely - feelings of alienation are part and parcel of being transsexual. And even if I had found a willing guy, this would not have been cool in Texas back in the 70's.
I mistook my desire to live as a woman, and my ability to relate to women, for attraction to women. Much like ReineD mentioned, my sex life was indeed lacking - I got very little out of it and in fact, by the end, I really did not want to have sex with my wife.
It's sort of a wonder I got married at all, much less twice, because a LOT of women read something different about me (some said this about me), and just couldn't be attracted to me. So when I realized at 18 that I wasn't likely to be able to transition, I decided to man the hell up, grow a beard, and get a girlfriend. (I'd managed to go through high school without one.)
So I got involved with my first wife to prove I was a man. I didn't think I was gay - I didn't understand that, either, because I definitely had crushes on guys, but they were all straight. Gay men I knew just did nothing for me - no interest on my part. I wasn't a gay man.
My second wife happened because, again, I was lonely, again I tried to be with a guy, and I just couldn't be attracted to any of the gay men I knew, and I really needed someone to help me with my young son from my first marriage.
After starting transition, I tried two more lesbian relationships, this time with trans women. Again, I was lonely, I'd always been with women, I still couldn't figure out how to get a guy, and most of us end up as lesbians anyway, so that's what I must be, right?
These relationships reinforced some things for me:
1. I hated sex with women. I got NOTHING out of it. I could mechanically do it, but it literally did nothing for me physically or emotionally. Nothing.
2. I hated being a lesbian! I hated lesbian bars. I did not identify with all the queer girls I knew. I didn't fit into that culture at all. I tried, but I hated it. Nothing wrong with it- but it just wasn't me.
3. For all the friends I made in the gay community where I live now, I really don't fit in here. I'm a straight woman.
So I started dating a guy, and I'll never look back.
BTW, I wish I had been a gay cisgender male rather than a straight trans woman. It would be many rungs higher on the social ladder, I wouldn't be spending thousands on transition related health care, and most people would understand what I was.
To recap: I married women to prove to myself and the world that I was a man. Proving I wasn't gay had nothing to do with it.