Quite simply
Mother Nature wants you to do it.
It makes it pleasurable, the more you have sex, the more likely you'll proprogate.
It makes you healthier, The longer you have sex, the more likely you'll proprogate more.
Nature does not take into account the "artificial" preventive measures.
It still thinks genetically we are the same as centuries before.
And so it promotes us multiplying.
And so sex is good in MANY different ways.
Only culture has messed up the definition of sex being good or bad,
but maybe this was developed in the past because it controlled over-population
and having only children that could be taken care of,
or prevented VD that couldn't be cured as well as now.
Now we have methods that are a bit more fool-proof,
not perfect, but MUCH better than past ones.
So the same social mores aren't necessary anymore.
And so we can go back to humpin' and bumpin' like our ancestors.
Have sex, be healthy & happy.