For this topic, men need to be thought of as lions. Lions are one of the only species in the animal kingdom who practice infanticide. That is when a new male takes over the pride (as it happens every few years) they tend to kill the young offering from the previous alfa male. This phenomena is the cause of 10% of all lion mortality.
So for our purpose, all we need to know is that men tend to find kids as a deal breaker. Not all, just those looking to start their own family. Their goal is to settle down and to be the patriarch of their pride. It is an ego thing. Why would they support another youth, be expected to take care, raise and nurture it better than the biological father without receiving any credit for their work? On a biological and evolutionary standpoint, this just doesn’t make sense. Especially if the child doesn’t call him ‘dad’. Because not only would the mother compare her new partner to her previous one, but the child, the mother’s family and friends would compare the biological dad with the ‘adoptive’ one.
Not saying that if you have kids, then you’re out of luck. Just saying this could limit your prospects.
Kids from other relationships tend to bring in tension. They tend to put the two adults against each other, especially if they feel left out or unloved. Not that blended families are not possible and or be completely functional, they are. Just the thought of being a part of a blended family, one that is not on his terms, is very difficult for the guy.
Guys are looking to be the partiarch of their own pride. They’re looking to find someone they can start a family with, not necessarily someone who will let them ‘join in’.