1) Biological facts - liberated wives

According DNA test of yellow-headed blackbird eggs, 45 to 65 percent of nestlings were not fathered by the social father. Females mate with subordinate males from within their social group, and the male partners play an active role in assisting a female bird to raise her young, contributing up to 60 percent of the work involved in incubating the eggs and feeding the offspring. Before that, courting, feeding the female and building the nest. Coupling with another male provides the female with high-end genes of her preferences and maximizes female birds’ chances of having healthy offspring. To assure this benefit the females have developed strong responses for cheating. The copulating lasted longer and sperm was found deeper in the ovary system. In a word, the female birds had more successful sex when cheating. If the introduced male was genetically superior to the social partner, with one copulation he could fertilize up to 80% of the eggs while one copulation while the social mate would fertilize by average 2.5% of the eggs. Copulation with alpha male is as good as 30 copulations with the social mate. “So even a little cheating can have big benefits,” he says.

The females, however, deprived their mates of access to other females by disrupting their extra-pair copulations and driving other females from their mates. When the females were successful in this, the social mates remained increasingly committed, since their genes only had a chance of being with their mates’ eggs. And with those males, in cases where the males observed the female coupling with other male, 80% of those males would still remain with the female.

Why? One could say since they knew their genes were not anywhere else, and at least they had chance (however slim) of being the father of the eggs they were hatching.

Those birds are not actually calculating odds or benefits. They just follow urges and instincts. Those birds have it right, right? They do it because it feels good and right, without guilt. I mean, it must feel better if it is 30 times more fruitful, or, maybe it is fruitful because it feels 30 times better. It works for the females and alpha males, but what’s in it for the cuckolded males? They are making a choice of behavior too, and actually picking the best option for them, so why wouln’t that be satisfying for them?

In human terms, your man is very likely in some ways like those 80% of the blackbird males who stay willingly even when they know what she is up to. Every male creature has a biological trait. To compete with other males and adulterous females, they have developed something called ‘Sperm competition’. It’s widely documented phenomena but I’ll give you the basics.

In evolutionary terms human mating system has always been something between a tight alpha lead pack mating system (earlier) and a free sperm competing system (later). In alpha pack the leader mates with everyone while other males watch, and if they manage to get their shot at it, they better be quick and efficient. In sperm competing system everyone mates but males with biggest testicles produce most sperm and have best chances. Everyone with millions of years with such history have adapted, and adapted well. For both systems, the main adaptation is ‘sperm competition’, meaning, men and males get aroused from watching or hearing sex, building up more fluids and momentum and persistency. Then they try to take these into action, displacing or overwhelming the seeds of others before and after them.

Now, most of the males (and females) would say that ‘yeah watching sex is hot, but…’ if it would be your woman in action with other male, it would be absolutely the opposite and not arousing at all. And yes in many ways it would be hard: when things involve emotions, expectations, assumptions and moral things do get complicated. But any man that says it wouldn’t be arousing – if the emotional part did not effect – is lying. Actually, if only the biologics were involved, it is the most arousing thing when your mate is involved. The body tells you must do everything to improve your chances with this one you have invested so much in. Most men will never admit that it would be arousing. They are defending their exclusive rights to their women, social position, religion, learned morals or values, and personal beliefs.

Well it can all be inverted if you know what you are doing. Let’s go back to the arousal part, according to Wikipedia: (They call sperm competition arousal cuckolding fetish)

The excitement and stimulation of the cuckolding fetish emerges from the biology of sexuality and the effects of sexual arousal on the brain. When a man witnesses or believes that his female mate may have been sexual with another man, the man mate is very strongly aroused, trying to end up in situation where he could copulate with the female, in an effort to "compete" with the other man's sperm. The effects of sperm competition are well documented.[4] The cuckolded man is prone to be very attentive and inclined on courting in order for allowance for sexual encounters. Meanwhile, the female enjoys greater sexual stimulation, first by her other lover and second by efforts of her cuckolded mate. Due other dynamics in such relation, the female is usually in a position to receive all the courtship, and to deny the act for as long as she wishes.

At early stage of any romantic relationship the female enjoys the neurochemical "highs" that include the effects of oxytocin and other neurochemicals which trigger excitement, euphoria and other similar feelings. Over time and as a relationship persists, the neurochemicals change to ones that promote bonding, planning and nurturing. However, with each new lover taken, the female can enjoy these exiting and euphoric highs, while going back to safe home to safe, loving and attending mate.