"The psychology of adultery has been falsified by conventional morals, which assume, in monogamous countries, that attraction to one person cannot coexist with a serious affection for another. Everybody knows that this is untrue."

"There can be no doubt that to close one's mind on marriage against all the approaches of love from elswhere is to diminish receptivity and sympathy and the opportunities of valuable human contact. It is to do violence to something which, from the most idealistic standpoint, is in itself desirable." [...] "but I do not recognize in easy divorce a solution of the trouble of marriage[...] but where there are children the stability of marriage is to my mind a matter of considerable importance.

[...]

I think that, where a marriage is fruitful and both parties to it are reasonable and decent, the expectation ought to be that it will be lifelong, but not that it will exclude other sex relations.

A marriage that begins with passionate love and leads to children who are desired and loved ought to produce to deep a tie between a man and a woman that they will feel something infinetly precious in their companionship, even after sexual passion has decayed, and even if either or both feels sexual passion for someone else. This mellowing of marriage has been prevented by jealousy, but jealousy, though it is an instinctive emotion, is one that can be controlled if it is recognised as bad."

Bertrand Russel, Marriage and Morals