Treat marriage as a legal contract
Possibly part of the opposition to persistent same-sex relationships is the fragility of opposite-sex ones.
While I was in South Dakota (1990-1998), a friend of mine went to Finland to be with his life partner (now his husband). They’re still together. Famously, when Newt Gingrich made protestations about the sanctity of marriage, he was asked, “Which marriage?” – of his three.
If the committed relationship of two persons for their lifetimes challenges the relationship of two others, perhaps the question should be about the relationship of those two other persons.
If the argument is about raising offspring, the logical consequent would be a prohibition against marriages that cannot result in offspring, either from age or from medical conditions (personal choice cannot be predicted).
Perhaps fertile, childless couples should be excluded from totally observant religious communities.
Ideally, the European model is perhaps better. The state does not rule on marriage in a religious sense. Rather, the state rules on a matter of contract law – since marriage originally was a matter of property law.
The state can rule on whether two persons competent to enter into a contract can enter into a domestic contract, with various property, personal, and tax rights flowing from the contract.