Not exactly medieval. And even in cultures were same-sex relationships were not an issue, usually you would still be expected to marry opposite sex.
Adoption is different matter, though i don't know how common it was for women to officially adopt. Formalized adoption (or many other things for that matter) was for inheritance reasons, and in patriarchal societies that was heavily centered around men.
However, and ofc this varied, adoption and marriage were less official than they are today. More about pact between partners and less formalized union, though state recognized marriages are definetly not a new invention. But nothing should stop you from making pact between two of you. It actually would be really immersive to have that kind of marriages as an option, they were very common in some societies. And in same way nothing should stop you from adopting someone and when you die you will continue to play as them - however, others might not recognize the union aka inheritance, which should lead to at least decreased relations/reputation, and if you are powerful enough, war.
Once game is more developed that could be really interesting role play to first play as "regular" character who builds reputation and maybe gets some estates, and after that character dies continue playing as your child who legally is bastard, and you would start campaign of seeking formal recognition. It would be really interesting way to dynamically create stories like that.
I don't like when people doesn't trying to find realism in combat or arms and armour instead. I want realism that could brings fun, good/natural/practical looks; like for an arms and armours, animations, combat mechanics/system.
Using the Gay Marriage mod, you can still have children but adoption is not an option. You have three options, your wife can get pregnant, you can get pregnant, or the default option is no children. Obviously two females can't really get each other pregnant, but you can pretend they have a surrogate.
Depends on culture and time...Octavian was adopted by Julius Caesar and became emperor of Rome...Tokugawa Ieyasu had his second son adopted by a rival for the role of Shogun...that's just off the top of my head I'm sure with research I could find tons more examples.