Sex is all about variation in your children
Life is about survival. Life is about passing as many copies of your genes on to subsequent generations.
Variation is the key to survival. The most genetically varied your children are the greater the chances of some surviving to pass on your genes to following generations.
I believe I read somewhere that 1 in 3 children in any marriage have a different father.
This is not surprising. Biology works that way and civilization is only a thin veneer.
Sex is about diversity. The two sexes have different sexual tactics.
As a biologist it seems to me that:
The female attracts in as many males as possible and selects the one she wants. Her aim is to find the most superior genes for her children. She works on quality. She is attracted to successful men. Her aim is to set up home, have children and have a partner that helps bring them up so that they can reach their full potential. While maintaining this nest she will look to mate with another male in order to bring a cuckoo into the nest. This gives her children more variation. Females can produce few offspring and go for quality and a small range of partners.
The male can father many children. He is less selective. He tends to go for quantity more than quality. He takes his chances where-ever he can find them. He will buy into the family and nest-building and raise his children but will quite happily father children in other nests.
The biological result of all this is that there is greater mixing of genes and a better chance of surviving.
It seems to me that the puritanism of religion has only ever served to mask what has always been going on. There is a biological imperative at work. It operates subliminally at a very basic level and supersedes morality.
Sex rules. Biology wins out. Even in the most repressed societies sex will find a way.