I've answered this twice, but i'll break it down as much as i can:

You need a female adult character.

You start the An Anthology of Sappho event that triggers on the top of your screen. This will make you a witch.

This will then allow you to do another event on the top called The Forbidden Ritual. This will convert your religion.

You are now able to marry women.

ok thank you sorry for asking again and again

just wondering what happens is I cant get a woman to marry?
IE the proposal thong is completely empty

It's possible they don't qualify for marriage, but it might be you have another mod that's causing some sort of conflict.

ok thank you

Step one. Get off steam discussions. Step two. Go outside. Step three...
Oh. Nvm.