A study on men in the Philippines recently demonstrated that being a "partnered father" substantially and significantly decreases testosterone levels - that same testosterone that significantly predicts whether a man will become a "partnered father" in the first place!
That same chemical that "predicts mating success" also is tanked by "successful" mating. As the authors put it,
these findings show that [testosterone] and reproductive strategy have bidirectional relationships in human males, with high [testosterone] predicting subsequent mating success but then declining rapidly after men become fathers.
How to solve this problem? Keep the sexual success (and the testosterone), but don't have the babies.