For one of Three reasons most of the time:

1: to protect yourself out of fear that you could very well be beaten to death because the fact that you are a man/woman who likes other women/men matters so much to some people in ways that it shouldn’t.

2: you think that if you ignore it you can live a “normal” life.

3: it’s an agreement between you and another person that allows you to play happy families whilst not having to be scared that your friends and family will not cut you out of their lives/you won’t have to live in fear of your children being bullied.

In short, people are cruel and it makes others desperate. As for the kid thing? It’s a lot harder for a same sex couple to have children, not only due to biological limits but because people believe they cannot parent children sufficiently like a heterosexual couple can and that the child will suffer in some way if they do not have a mum and a dad. Adoptions and surrogacy is also insanely expensive so the route some people take is to have a child with one their friends and deal with the problems that come with it later.

Again, we don’t know much about op’s marriage in General so we can’t exactly say why she did what she did, but I get the feeling that it was done out of an attempt to live the “normal” life she believed she wouldn’t get if she pursued women.