generally they don’t enter into contracts.
contracts for commerce are one thing.
marriage is an institution.
akin to voting rights.
They may never have be adjudicted incompetent enough on THOSE specific issues.
Marriage is an absolute legal contract that comes with tax benefits, survivor benefits, etc.
There is a religious aspect to it, but there is also a state issue to it. If they are ok getting married, they should move out and live as a couple.
This is what happens when folks go all PC and fuzzy wuzzy. Legal actions have legal consequences.