Terry Gould’s book The Lifestyle also makes the point that sperm competition is one significant factor in MFM activity: when you suspect (or in this case see) your female partner with another man, your body goes into overdrive in order that your sperm should compete with his and fertilize her in preference to his sperm. So your erection is more urgent, your semen more copious, your energy greater, and your sperm count much higher. In other words, it’s a boost designed by nature to enhance your probability of passing on your DNA. Today we can consciously use this as an erotic stimulant.
That said, in my experience only a few husbands/boyfriends can handle the emotional consequences, especially in the sexually anxious USA.