That seems to be quite a blanket statement there -- there are plenty of husbands who don't want space and don't want to get into other relationships, as well as plenty of wives that do. At any rate, to the question:
I think that in a committed relationship, there is often one person who feels somehow entitled to their partner. They feel like they have the right to control what the other person does, or that they have the right to all of the time they want from that person. The one that is being controlled will often withdraw in an effort to keep from being suffocated and to be allowed to be their own person. Someone who tried to control has lost their respect for the other person as an individual, and I personally wouldn't blame anyone who tries to get away from them.
On the other hand, in some relationships, once both partners get to know each other better they realize that the other person isn't really who they thought they were, they don't like being around them very much, so they withdraw.