There are a number of reasons men have for wanting their wives to have sex with others. Each man varies in what aspects turn him on. One reason is the idea of seeing their wives having sex. When a man is actively involved in making love to his wife he really can't focus on how beautiful she looks and moves. Think of you starring in his private and personal X-rated fantasy.
Another reason is the idea you being sexually attractive to other men and desiring sex for the pure thrill of sex. Sometimes it is hard for a man to see his wife in this light when she is also "wife" and "mother".
Sometimes men have a masochistic side hidden in their fantasies that desires his wife to find him less sexually attractive or capable and for her to discover the joys of bigger and better men.
There are many other reasons and he may not fit exactly into any one of these areas. It is said that men are ruled by the little head and not the big one, but as men get older the little head needs a lot of help from the big one to stay active. In plain words he needs more fantasy in his life to get sexually excited. Read up on this cuckolding thing, and disregard anything you think is too gross. You can make your man happy for years just by teasing him with the idea, or the possibility that you might have done something in the past. Lastly, remember that a many a man's desire for this fantasy tends to disappear after he climaxes.