in a secular society, we don't generally write laws regulating who is allowed to have sex with who - we leave that to the discretion of the people having sex, whether they're married or not, and of whatever gender combination - so long as the consent is legitimate, and possible to provide for.
so, you can only marry one person at a time, which is a kind of a legal formality, but that doesn't mean you can't have relationships with multiple people if you want. that is a freedom we have in this society, so long as we're willing to accept the legal consequences of cheating on our spouses regarding questions of alimony and child support.
i'm just pushing back against the idea that reversing the sexual revolution is some kind of feminism, because the situation of course works the other way as well - women should not be legally punished for cheating on their husband, which is probably the more pressing issue to concern ourselves with in terms of consequences. that's a freedom we have in this society, so long as you're willing to accept the legal consequences of the subsequent divorce.
what we do, instead, is provide people with the freedom to dissolve the contract - to walk away if they don't like what their partner is doing.