To avoid confusion, you could have two people who wanted to have sex go before witnesses and declare their intent to cohabitate, have sex, raise children togeth

To avoid confusion, you could have two people who wanted to have sex go before witnesses and declare their intent to cohabitate, have sex, raise children together, etc ("vows" I guess you could call them, although that sounds a little bit medieval). There's no need for it to be stuffy though, you could wrap it in a big party and wear fancy clothes, read poetry, play music, all that fun stuff.