So my question is this: Is a woman who has sex with Brad Pitt once and remains alone for the rest of her life actually happier than a woman who is comfortably married to an ordinary guy for several years?

You make a compelling argument. I clearly misused the word rational when I was just looking at what the genes "want". I thus retract that part of the statement.

I do wish to emphasise that "5 minutes of Alpha is worth 5 years of beta" while mostly hyperbole is something people should be keeping in mind when trying to predict the sexual and romantic behaviour of women.