Yes, men should, in addition to being open minded about women who are "sluts," is if the relationship is getting serious (or even if you’re already married) is to allow and encourage your lady to play around while remaining monogamous yourself. Show her your generosity and the fun side of sex.
Men, keep in mind that a woman, your woman, unlike a man is capable of having sex with someone else then performing and giving you perfect satisfaction.
And it is wonderful.