Nature prefers genetic diversity. Reservoirs of genetic variants that might save the species whenever the dominant variant can no longer adapt to changed circumstance. That's what the male sex was "invented" for. Females have a smaller set/lower change of mutations whereas daily male sperm production (with more mutation chances as the male ages) insures way way more mutation changes. This works hand in hand with male promiscuity to maximize heterogeneity in any given gene pool.
It is a fact that even some "monogamous" bird species actually involve females that fool around
