For evolutionary reasons men don't want to support someone else's kid.
Don't blame evolution for it; The reasons for that attitude have
nothing to do with evolution, and everything to do with societal rules and ideas mostly defined in religion, and designed to ensure inheritance of property, wealth, and power.
Indeed, the exact same thinking that says "I don't want to raise another man's kid" leads to "The royal bloodline is special and sacred", with the consequence that many dynasties have suffered horribly from genetic disorders resulting from inbreeding. It's an idea that
reduces evolutionary fitness, and which in the long run (and evolution only works in the
very long run) is doomed.
Evolution gets the attention of a lot of ignorant people who have finally realised that "the will of the Gods" is no longer an effective claim.
But evolution works at the population level. Humans have NOT
evolved to be disinclined to support the children of others; They have
decided to do so, as a means to wealth and power. And they made that decision very recently - far too recently for evolution to have been in any way relevant.
The attitude you ascribe to evolution has existed for fewer than two hundred generations, and developed through a time of rapid population growth (which inhibits evolutionary pressure, because during such growth,
many successful reproductive strategies must exist for a species such as ours with a low rate of reproduction).
Evolution doesn't operate on our understanding of how we reproduce; We
evolved in an environment in which the link between sex and babies was very tenuous indeed.
Evolution doesn't tell us how we
should behave. It's not a set of rules. It's a description of our history. We are entirely at liberty to ignore it, and more than capable of doing so - and refusal to support other men's children is an example of so doing.
If you want to see what nurturing behaviours evolution is responsible for, take a look at how we behave towards children who cannot be inheritors of property and power, and are at best only distant relations of ours - because they are not human. If "For evolutionary reasons men don't want to support someone else's kid", why do men want to support puppies and kittens?
Men don't want to support other men's kids because society has told them that doing so is demeaning and unnatural. The former is artificial (and circular logic - you should be ashamed to do it, because it is shameful), and the latter an outright lie. Evolution has exactly nothing to do with it.