For those of you interested in cuckolding, hotwifing, and other similar fantasies, this is an excellent post on the subject. The author also writes a pretty fantastic blog called The Psychology of Human Sexuality; I recommend you check it out!
His post gives nice space to discussing sperm competition theory, which I love:
“Increasingly, scientists favor a biological explanation based on a growing body of work on sperm competition. Research shows that when one woman mates with several men, those men can display behavioral and biological changes intended to increase their likelihood of fertilizing her egg—without even realizing it. For example, when men masturbate to porn featuring multiple men having sex with the same woman, their ejaculate contains more active sperm than it does when they beat off to an all-female threesome, according to a 2005 study of 52 men. Other research has found that men report thrusting faster and deeper during sex when they suspect their female partner has cheated, presumably as a way of displacing rival sperm. These findings suggest the provocative possibility that men are “wired” to find cuckold scenarios arousing because they promote behaviors that help their own sperm win a raging intra-vaginal sperm war.”