I keep thinking about those women who have sex with one of their best girlfriend's ex. Or, rather, I keep thinking about why this offends us so. I don' think we're wrong to think this. I especially don't think the woman who discovers that one of her friends had an affair with her ex is wrong to feel that way. That said, coming up with good reasons to justify the feeling of being offended is difficult.Mikolaj gave an interesting response to that in the comments to that earlier post:
That's easy to explain based on the assumption that men are polygamous and women are hypergamous (we are talking about premarital sex, so this is post-Christian, so monogamy is no longer a frame of reference). Obviously, in any relationship, the man's polygamy and woman's hypergamy cannot be both fully realized, so this is a struggle that often leads to the dissolution of the relationship.I think that is an interesting possibility and I read variations of this argument on a lot of men's sites. I don't think it works. There are a couple of factual issues to start with. The first is the assumption a woman's best girlfriend is bound to have a similar status. In my experience that is almost never the case. The second issue is the point I started with women having sex with their best friend's ex. I didn't ask about men who do this and I didn't for a reason.
Case 1. He, being polygamous, dumped her for variety. The best girlfriend helps him obtain the variety, even if she is not the first women he sleeps with afterwards.
Case 2. She, being hypergamous, dumps him for a guy with a higher status. The girlfriend proves she was wrong assesing his and her relative status (best girlfriend is bound to have a similar status).
All my life I've heard women complain that someone they thought was their best girlfriend moved in on their ex really fast. I was in two serious relationships before I got married and when I broke up with those women I found myself being "consoled" by some of their best girlfriends, in one case before the actual breakup. (I'd like to say that I turned down all these offers but I took up two.) I don't hear similar stories from men. And when I read websites aimed at women and I usually find an article about how doing such a thing is a horrible betrayal to a friend. You don't find similar warnings on sites aimed at men. I think that tells us something important: people only put up signs up saying "keep off the grass" when they know that people are already walking on the grass. It's telling that women feel the need to put up a sign about this particular stretch of grass and men don't.
I think Mikolaj is right that the explanation lies in evolutionary psychology but I think he is looking in the wrong place. First of all, pair bonding is not an artefact of Christianity. It's an evolutionary strategy. If a man and woman stay together, the chances of their children surviving go up. Another evolutionary strategy is promiscuity: a) when a man has sex with a lot of women he increases the number of children he has and therefore the odds that some of them will survive and b) when a woman has sex with a lot of men she increases the chance that she will get pregnant. With humans, pair bonding tends to win out because we are capable of forming complex social relationships that further increase the survival possibilities of all the children in a community. However, promiscuity never goes away: you always get free riders within the system. And, as much as this may wound our male vanity, women hold a lot of power here. The key biological fact that evolutionary psychologists point to is this: no matter how many men a woman has sex with, if she has a baby she knows it is her baby. Men don't have that assurance and are, as a consequence, vulnerable. Some females (and this is true for all species that pair bond) will attempt to have sex with a high status male who is not willing to commit to her and then try to fool a lower status male who is willing to commit that it is his baby by having sex with him as well. She can't actually be sure who will impregnate her but there are behaviour patterns that will promote that outcome (pity sex comes from somewhere). None of this need be conscious: she just does what she feels but her feelings are strongly influenced by evolutionary biology.
Now, you'll have noticed that I've substituted "pair bonding" for the terms "monogamy" and "polygamy". I did that for a very simple reason: harems have guards. The high status male is just as vulnerable as the low status male. A very high status man may accumulate a number of partners but those women can only pass on their genes if he can get them pregnant. In populations of wild sheep, you get cases where powerful males are able to accumulate huge harems but are unable to perform sexually at a level to get many of them pregnant. You can sometimes improve the survival opportunity of wild sheep populations by killing off alpha males. To bring it back to humans, even in a polygamous society, women will have a powerful incentive to cheat. In this case the high status man is the one who can actually impregnate her and the low status man can be the one who appears more "alpha" from outward appearances.
As Hobbes says, even a weak man can sneak up behind a strong man and hit him on the head with a club. Similarly, even a low status male can move in on a high status male's partner and it gets much easier for him to do so in a polygamous society. Look at actual polygamous societies and you'll notice that they are the most patriarchal societies on earth: think Saudi Arabia. Paradoxical as it might seem, it is only in a culture where monogamous marriage is the generally accepted ideal that sexual freedoms for women are possible.
The woman's power is not unlimited; the problem she faces is getting caught. She has to be sneaky about it. She may accept a non-exclusive relationship in order to get pregnant but she needs someone to support her and her child. That can be worked out in various ways: polygamy, government child support payments or by fooling some guy into thinking he's got an exclusive relationship. Remember that we see polygamy fairly often in human relationships but almost never polyandry. Men will only accept non-exclusive relationships in the case of real or perceived scarcity. (By "perceived scarcity" I mean a man whose self-esteem is so low he doesn't think he could get sex any other way. Such men exist but everybody, including the men themselves, disdains them. The classic blues song "Back Door Man" may sound like bragging but actually it's the sort of pathetic pleading men do out of resentment at being at the bottom—there's a reason we find blues and rap songs on this theme.)
None of this actually answers my question but I think it points in the right direction. To flip the initial claim around, odd as it may seem, polygamy is likely only if we restrict polygamy to the strict meaning of one male with multiple female partners. What I mean by that is that women will accept to enter into a relationship whereby they share one man with more than one woman and a man will be willing to support more than one woman even though that increases their vulnerability in some ways. They won't prefer it but they'll do it. That isn't to say women will like it, they're just more likely to tolerate it. Men will hardly ever tolerate polyandry.
As a consequence, when a woman senses that her girlfriend's relationship is foundering, she is going to think about moving in. It's a tricky play and it won't actually happen in most cases but it's always a possibility. There are risks. The woman who actually does sleep with her best girlfriend's ex has to be sneaky about it. She will pay a serious social cost in that she will lose friends and social status if it gets out that she did this. That said, women are better at being sneaky about sex than men precisely because they have more to lose if they get caught. She isn't planning on telling anybody and she can be reasonably confident, given the circumstances, that he'll keep the secret too. (Remember that it's usually the woman who ends it so he is probably hoping to get back together even as he has sex with her best friend—that's a powerful incentive to keep a secret.) She's had lots of time to get to know this guy and she shares her best girlfriend's taste in lots of other things so probably men too. In addition, the guy's ex has probably been bad-mouthing him to her in ways she knows aren't fair and that might make it seem almost justified.
Men, meanwhile, are going to be pulled the opposite way. A recently single woman is going to suddenly feel less attractive to them because we are programmed by evolution to avoid situations where we might end up raising another man's child. It happens of course. I've known men who have pursued a series of relationships with women who are on the rebound. But only a man with very low self-esteem would pursue such an option.