Sometimes, a wife can react badly to the suggestion she sleeps with other men, but a year or two later, she can start doing it behind your back if she finds you neglecting your marriage.
She can discuss what you suggested with her best friends, and find out that some of them have thought of sleeping around (or actually are are currently sleeping around) and it can grow less distasteful from therein.
Some of the most prim and proper wives can "swing over" as revenge if opportunities present themselves - but a husband can hint she has his permission, but "pushing her" can make her feel that her marriage vows are being destroyed.
A wife needs a good reason to go outside her marriage vows, but first she has to feel that her marriage is strong and won't fall over no matter what happens. If her friends are fucking about, then she is more likely to consider doing the same sometime in the future, and will enjoy hearing the gritty details of other wive's adultery.
Mills and Boon writers are slowly introducing lesbian scenes and oral sex into the latest books hitting the shelves. What was once taboo, is now talked a lot about by modern women. Adultery is not always the "end of a marriage" any more.
Even Tiger is holding on at the moment, even though he has been publically humiliated. Billy Clinton seems to have survived, and lots of women check him out to see what Monica liked about him enough to drop her skirt.
Sex and the City is also loading women's minds with thoughts of adultery or Open marriage or Swinging or inter-racial. A husband has to "not push her into it", and he has to think long and hard because it is definitely cruel to talk her into sharing her body with other men, and then pull back and tell her he can't stop feeling jealous now it has become reality.
If you want your wife to fuck other men, NEVER EVER CHANGE YOUR MIND, or she will tell her friends you are an unfeeling bastard who don't keep your promises. Sex with other men is a gift you offer her - you can't withdraw your gift without hurting her emotions.
A women has to churn "your permission" over and over in her mind for months before she may do it TO PLEASE YOU", so be sure it is what you want because when she starts dating other men, she opens up a pandora's box and will discover her hidden sexuality that has been compressed by her moral upbringing. She can't turn off the new emotions and desires, just because the Bull invades your marriage in a way u never envisoned.