Someone: "Guys' memory with respect to previous sexual liason brings the following benefits:
1) Knowledge of the partners' sexual preferences, so one may employ this knowledge to procure future advantages.
2) To differentiate between one's many sexual partners, so as to prevent embarassing, and potentially reputation damaging, moments.
3) So as to know who one may look for in case one needs help (reciprocal altruism).

Me: 1) I suppose this will only apply to medium/long-term mating? STM is indeed just a one shot thing, so there'll be no point I think. But then in M/LTM women go for factors other than sexual skill, so...

3) "Hi. I had sex with you last week. Can I have a favor now?". Erm.

Someone: Gabriel, may I say that you have no experience of any night life?

1) Men and women who look for casual sex in contemporary societies usually hang out in the same few areas. Even for those who do not frequent the usual pickup areas, they get their booty calls from a relatively small group of friends.Memory would definitely help.

Furthermore, as I mentioned, ancestral men lived in small communities. Memory would help them, and since we are the products of our ancestors, we would have such memory too.

Plus, if you do a little thought experiment and try to conceive yourself picking up a girl for casual sex, ask yourself if you would remember her name, or at least her face. Probably yes. Unless you have casual sex every night. Which I doubt so, for most swingers have, on average, about three sex partners per week, and these are extreme cases (most swing once per week/fortnight).

3) Why not? Most swingers have their community. Swingers introduce their friends to each other, and hence establish "sexual networks". Sounds like reciprocal altruism to me. Furthermore, it can be observed that females usually go up to guys they had sex before to solicit drinks.

Me: You learn something new everday.