It might, conceivably, work for a species that pair-bonds but without any other significant social structure, but for a species that's spent most of the past half-million years or more living in tightly-knit bands, the relevant selective forces are likely to be rather different, and even cultural differences should exert a significant force on the way these instincts are expressed.

I don't understand this objection. Doesn't a social structure increase the value of sexual reserve as a resource? Notice that it does so for both women and men (though more so for women), as wronged women can now get back at the people who wronged them more effectively.

Regardless, let's look at the basic argument. There are two basic strategies suggested for Sally: the strategy of having sex with anyone she wants to while trying to maintain her sexual health, and the strategy of not having sex until she has extracted a public, permanent commitment from someone she wants to have sex with. (Those are extremes- some combination of them is also possible, like the 'don't have sex for 90 days' plan, which seeks to demonstrate sexual reserve without requiring the permanent commitment.)

What factors should she take into account when determining what strategy to pursue? It very well may be that she cares more about having enjoyable sex than having a long-term relationship, but humans are also pretty bad at knowing what will make them happy. It is unsurprising that her parents are giving her advice that will maximize the chance of grandchildren whose father is invested in them, and their advice should be taken with a grain of salt if trying to maximize her happiness.

But the main thing I was trying to inject into the conversation is the knowledge that, whatever baggage she has (be it genetic or memetic), Bob probably has similar baggage, and she might want to plan for that. If she needs to work through some negative emotions, she shouldn't be surprised if Bob also has emotional issues he needs to work through, and that some things are harder to work through than others (it may be easier to overcome guilt than to become more attracted to someone, for example).