I was thinking about something. So let's see a girls sees an attractive stranger who wants to have sex with her. Why wouldn't she go for it? The only major reason from evolutionary point of view is that after sex the stranger might leave and she might end up with a baby and nobody to look after it, what would be horrible for her. But - if she already has a boyfriend, this reason disappears completely.

So by this logic, girls are far more likely to have nsa sex with you if they have long term boyfriends than when they are single.

Biologically, I cannot find any flaw in this argument. And it goes against pretty much everything I've heard about relationships, from both PUA and traditional sources.

Does it work like that in practice? Real properly controlled research is sparse, so anybody wants to share their experience here?

The endocrine system probably explains a great deal of the apparent discrepancy. Long term relationships promote a cocktail of chemicals that foster attachment. In a satisfying long term relationship, both women and men (women, probably moreso) develop neurochemical responses to their partner that generally make monogamy a more comfortable experience. They psychological upshot of this is that a woman in a long-term relationship doesn't just want to mate. She wants to mate with her partner more than she wants to mate with other men. Her inhibitions about the costs of pregnancy are lowered particularly for her partner.

This stands to reason. An attractive stranger has not already proven himself a suitable partner for a long-term relationship. Her current partner, therefore, has the advantage of established value. Yes, women DO cheat, so obviously this isn't the only factor. I'd hypothesize, however, that women cheat under the same kind of circumstances under which they'd have a one-night stand: i.e. when they're NOT thinking about the costs of pregnancy. Correspondingly, I would hypothesize that a woman who is actively considering the costs and benefits of pregnancy does so mostly with her most suitable long-term partner in mind.

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Some vices miss what is right because they are deficient, others because they are excessive, in feelings or in actions, while virtue finds and chooses the mean.
Aristotle, Ethica Nichomachea

There was a study by David Buss I believe. He asked the question,

If you had to pick what would hurt more
A. your partner falling in love with someone else
B. your partner having sexual relations with another person

From his study, most men answered B. and most females answered A. He said this was because females want a man who will take care of them when they are pregnant, but if they are in love with two people, then his resources will be divided between the two. And males are constantly worried that their resources will be used on a child that isn't theirs, so her having sex with someone else increases these chances.

So I think women may one some level may understand that if they do have sex and get pregnant they could be left alone because the one night stand won't stay around, and depending on the guy he could be gone too.

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Men fall in love with their eyes. Women fall in love with their ears.