> I would then ask for proof that family is indeed the "basic building block of society"
I think the burden of proof goes the other way, considering marriage laws (including those that punish infidelity) have been part of common law for hundreds of years.
Have you seen a society of random people who don't have long-term relationships? A country which laws don't treat family as a basic concept, and third of which is not built around it?
That family is a fundamental unit in human societies is a pretty much self-evident thing at this point. The burden of proof lies squarely on those who disagree with observable reality.
Malinowski [0] describes a highly functional such society. Instead of paternity and monogamy, they had a similarly complex but different set of taboos. Reading it will help you realize what a cultural bubble we all live in, and that there are completely different configurations of society that also work.
Families exist regardless of marriage, marriage != family, so to claim that attacking marriage tears at the foundations of society and then to move the goal post and say they're attacking families, your must first demonstrate what you're saying is true. Marriages don't make families, loved ones do.
I think the burden of proof goes the other way, considering marriage laws (including those that punish infidelity) have been part of common law for hundreds of years.