The sad truth is most men are disposable (from an evolutionary point of view), much like the vast quantities of sperm they produce.
Plain false. Humans are very unique compared to other animals as I wrote in another response. See prof. Sapolsky's lectures on the topic. He's an outstanding anthropologist.
We don't change our instincts just because there are laws. Look at how many people are in prison for drug possession. The law is much weaker than brain chemicals being expressed. (Also, not every law is based on wide agreement of human emotion. Sometimes lawmakers simply do whatever. I'm mostly talking about the ones that have been around since the dawn of humanity.)
Gender roles (and sexism) are primarily cultural. If you look through history there has been matriarchal and patriarchal societies. Monogamous and polygamous and so on in many combinations.
Humans sexuality is very unique compared to other animals and, respectfully, we should try not to play armchair Darwin.
There are very good lectures from prof. Sapolsky on youtube on the topic. Humans are complex and very weird.
I think, slut shaming might have a biological component: you can't be shure the kid is yours.
You can have a stable/growing population with only 1 male per n>1 females. So, the evolutionarily optimal strategy is to crank up trait variance in males and have females select only the top 1/n males (hand-wavily speaking). Most modern human cultures suppressed this dynamic via socially enforced pair bonding. When you lose that social adaptation, you revert back to more atavistic matching dynamics.