How can one possibly be expected, nay, commanded by morality and reason, to deny one's most beloved person a pleasure for the purely selfish reason that it doesn't involve oneself?
Why should I be jealous? What is it that is detracted from my husband if he has sex with another woman? Is he now a different person? Has he changed physically or emotionally? Had he given away a bit of love that can now never be mine?
I am incapable of jealousy on purely sexual grounds. I can, very occasionally, be jealous of my loved ones' time, of their affection - but of their bodies? What reason on earth do I have to be that?