So we have our modern setup which is more or less sexually permissive. "But wait," you might say, "it doesn't matter if men want to paternally invest or not. Child support will make them!!!."

Sure, you can try to force men to invest through child support etc., but only a little over half of ordered child support is ever paid, despite the stiff penalties.

Furthermore, child support is arguably a clinical, bare minimum kind of investment, and ostensibly many men are actively avoiding being productive members of society in order to avoid incurring support obligations they (rightly or wrongly) perceive as funding sexual permissiveness.

not to mention men avoiding playing a part in their children's lives (investing time, etc.)

Now, yes, it's true that incentives don't play out in a universally identical manner.

I'm sure your uncle Frank happily invested in and raised his wife's extra-pair babies...maybe he even encouraged her to go get some strange! Some couples adopt. Etc.

and intelligence can affect the degree to which people are able handle or negotiate these kinds of scenarios

But on the aggregate, the more sexually permissive the environment, the less likely men will be to want to parternally invest.

In short, I think we are currently in an unsustainable middle ground that tries to have the best of both worlds/have its cake and eat it too.

If we're going to have sexual permissiveness, the society needs to be structured around the expectation that the mother and her kin (including male kin) will team up to raise and provide for the baby.

This is what happens with the famously promiscuous Mosuo, and is swiftly becoming the de facto child rearing arrangement in working class households anyway.