me too
I don't think there'll be many mangas like that, considering that they are from Japan where same sex marriage has not been legalised. You can read webcomics, though, from sites like tapas or smackjeeves where most of the artists are from countries where same sex marriage has been legalised.
I would like to know a manga of your description too! :3
New York New York is like that. It's probably one of my favorite.
First thing I thought of: http://www.mangago.zone/read-manga/new_york_new_york/
They're both openly gay, living together and eventually they get married and adopt a child ヾ(❀╹◡╹)ﾉ~
Umm, you could try reading omegaverse ones since there they live openly gay and marry but it often has rather more sex scenes
http://www.mangago.zone/read-manga/kashikomarimashita_destiny/
http://www.mangago.zone/read-manga/kurui_naku_no_wa_boku_no_ban/
http://www.mangago.zone/read-manga/tonde_hi_ni_iru_natsu_no_neko/ (not omegaverse)
http://www.mangago.zone/read-manga/sweet_heart_trigger/ (also no omegaverse)
http://www.mangago.zone/read-manga/mou_ichido_nando_demo/ (no omegaverse)
http://www.mangago.zone/read-manga/irony_dress_ni_sayonara/ ( no omegaverse)
http://www.mangago.zone/read-manga/tadaima_okaeri/ one of the cutest omegaverse that exists, they have a child
and this is also a cute omegaverse http://www.mangago.zone/read-manga/kasegimasen_kara/
Hope that it helped you a little (I found some non-omegaverse while I searched)
Oh and maybe this one too http://www.mangago.zone/read-manga/asa_to_mitya/
here they focus more on their relationship instead of what others think
I don't specifically collect openly gay couples but you'd find a lot of them in my "out of the closet" list.
also, here's one that http://www.mangago.zone/read-manga/wagamama_romeo/
I want to read yaoi in which there is a gay couple, openly gay, living together, maybe planning to get married or already done it. I Just want to read a yaoi in which there is no drama of the sort that the one is gay and the other is straight, married or his brother.