Relax. Understanding our primal instincts doesn't give us excuses for breaking higher moral codes. Those codes also have a primal base - pair bonding is found in many animals apart from humans. Generally it occurs where there is a long gestation period and the offspring require both parents to ensure they survive.
Humans have the longest maturity development of all creatures - about 10 years, so its vital for a mother to know the father of her child will stick around to protect and feed both. Equally the father will stick around because he wants his genetic offspring to survive.
A sexual relationship with another woman is considered cheating. From an evolutionary perspective, that is explicable. If a man can create more children, its a win for his genes - but it also means divided resources. Now he has two families to support. On the African savannah with seeds, berries etc plus small animals for meat, the pickings were slim so supporting one family became primary.
None of that means a man will not cheat - if he can fool another man into raising his child. So men will take the opportunity if it presents itself. Which is why easy girls are popular, but not as potential partners - you can't be sure who's child you'll raise.
Bookmarks