Numerous biological factors stress that humans are meant to be monogamous. The way the penis bends supports sex face-to-face which is different from nearly every other species, and face-to-face sex establishes emotional bonds. Our oxytocin levels increase the more long-term relationships get. Human females have a menstrual cycle opposed to an estrous cycle, and show no visible signs (to men) when they are ovulating, and can have sex all throughout their cycle, not purely for reproduction (argued by anthropologists that it keeps a man guessing and keeps him by a woman's side). Oxytocin and prolactin surges in newborn's fathers as is does mothers, which facilitates the bond between father and child, and thus father and mother, etc etc