Straining against monogamy of course is the genetic advantage for a male to "spread his seed".However even if having children with other women if given the chance, he may likely concentrate on the children with one woman.There can also occasionally be a genetic advantage for a woman to stray.She could get pregnant by what she or her body tells her is a high genetic worth male but return to her first mate to raise the child- he may be a better parent and less likely to stray himself and he might never know the child wasn't his but raise it as his own.