Dark heaven?
Concerning marriage there is
Dark Heaven
http://www.mangago.me/read-manga/dark_heaven/
Hare no Hi
http://www.mangago.me/read-manga/hare_no_hi_asada_nemui/
But most seem to centre around "adult adoption" rather than actual marriage.
Example http://www.mangago.me/read-manga/hana_no_migoro_ni/ From the hana series.
I don't think that fits the category. It's too dark (no pun intended )
It ends with an actual wedding/marriage though as opposed to "adult adoption". The OP didn't specify that it had to be fluffy, but I would say the series does have it's sentimental moments.
guys dark heaven is way too depressing
That's omega land, I thought the OP was looking for more realistic depictions of marriage and adults adopting children of their own (or using IVF) which would mean the setting would have to be international. A lot of manga's that are set in Japan depict "adult adoption" rather than legal marriage which is obviously reflective of the homophobic laws in that country.
The OP is looking for romance and family theme! I am pretty sure they don't mean a story revolving around Racism, Homophobia, Rape, Crime, Prostitution and Character deaths.............. (I absolutely love Dark Heaven!!! It's beautiful! But it might not be everyone's cup of tea )
Such is life though, and given the origin of manga, most do not depict actual marriage therefore the options are limited unless you want to go into the pure fantasy realm of omegaverse. To see Japanese authors tackle marriage in a realistic manner, rather than only depicting two men being forced to legally be "husband/father" because their country does not have equal marriage is relatively progressive. The OP can skip to the end if all they want is a church wedding.
http://www.mangago.me/read-manga/gochisousama/ no marriage but the seme propose the uke
http://www.mangago.me/read-manga/kanga_na_spoon/ it dont show the beginning before they become a couple
http://www.mangago.me/read-manga/wild_rock/
http://www.mangago.me/read-manga/ai_no_bousou_ressha/
You can't have all of it together. I think for family theme , one of them would already have a kid or be raising their siblings kid . If you want to check it out, search Yaoi manga with children on Google and choose mangago's link. And marriage is super rare, you can only find it in omegaverse and those sacrifice to demon mangas.
Is there a yaoi manga where the guys go through the whole process of falling in love, proposing, getting married, and having/adopting a child? i really want to see the full cycle