Stringsinger I slightly disagree with you though you have got what I meant. You said it better :-)
I would saying being in love is the stable relationship and when coupled with sex is fine. What I meant was the chemicals of faling in love and sex. It's a puwerful combination. However, your point is taken and I agree with what you mean
It's so complex! :-)
As for Human's having estrus, in some ways females do go into heat (some at least) close to ovulation. Theit ability to smell certain pheromones, be able to tell symetric males better, and to orgasm, are on average increased. The chance of her getting pregnant also increases if orgasm is achieved (where no contraception is used, obviously).
Sperm competition patterns differ in different animals and it is that which drives whether monogamy can exist or not. That said, even some animals considered monogamous do hedge bet with extra pair matings and humans are not above that either!