If that would be true then there would not exist any pair bonding species, and there is some excellent benefits from it. Two parents mean one backup while raising the children, and two parents is double the parenting. Two parents also give some potential benefits with twins, something which pair bonding species tend to have a lot of while non-pair bonding species don't. The primary argument in favor of pair-bonding is that having a lot of offspring that don't reproduce is worse than having a few that do.

Humans with our great variations of behaviors has both pair bonding behavior and non-pair bonding behavior, with genes that code for both. If one strategy was always superior to the other then we would have settled for it a long time ago.

(Its a law or something when talking about genes that the behavior is always a gene-enivorment interaction, and there are likely many more factors that influence pair bonding behavior).