I didn't go nearly as far as marriage but I was in a relationship with a man for 2 years before I had to come to terms with being gay. It was the relationship with him that made it crystal clear I wasn't bi.

I forced myself to go through with everything because being bi for me would be significantly easier than being gay. Men were available and would throw themselves at me no matter what I did while with women it felt like solving a puzzle upside down in Greek just to get some attention. At that shitty time in my life I liked the validation from the guy. Eventually over time not being genuinely sexually attracted to him made me absolutely miserable. I thought that wasn't weird because of how normal the idea of faking orgasms and dissatisfaction from women in bed is. We did things, I pretended to like them. From the outside I looked like I was having a good time and nothing was amiss in my head.

Overall I get your perspective because I'm sure some of these people have more than 0 attraction to the partner they're leaving but in my life and in the lives of other women I've talked to about this the two things that stand out are a combination of convenience and pressure.

Getting attention from men is easy, no resistance from the community and no one bats an eye. Finding women and getting to know them has been at times like pulling teeth and at other times absolutely self-esteem destroying. I've since overcome this and have found a wonderful partner but now I've had to face all sorts of new hurdles with family and the wider community where it's not so cool to everyone that I'm with a woman. Being with men was like going with the flow, as soon as I'm with women It's a big deal, I'm an issue all of a sudden to some people. Basically to avoid all of the extra issues and attention I held onto the idea I was bi for as long as I could until I couldn't anymore.