From a genetic dominance standpoint, the female has to live with the consequences of sex...for a LONG time as human children are totally dependent on their mothers
for a period of several years and continue to be dependent on parents/family group for several more. Women have a natural instinct to select a GOOD male to mate with.
(Apparently, this natural instinct is very fucked up in many cases, considering the kind of turds some women will hook up with.)
From the male perspective, we don't HAVE to be saddled with the kid, and it is to the benefit of our genetic line to sow the seeds far and wide. We are naturally inclined
to jump fences and hump what lies beyond them.
Just be glad that our paternal instincts aren't as strong as with some animals. For example, if a new stallion defeats the old stallion that dominates a herd of horses, and
chases him out, thus winning dominance of the herd, that new stallion may (and often will) kill the newborn and young foals sired by the old stallion, in favor of his own foals,
which he will be siring with his harem of mares as soon as they come into season. (If you actually want to see that happen, there's a video on youtube titled "Stallion meets foal"
which shows you that this in fact does happen.)
Several other species are known to do the same thing. Lions, for one.
We don't have any instinct to kill the children of another man. Be glad for that, kids!
....and women!