Culturally, the expectation is monogamy. Now there are women and men who will cheat for a variety of reasons. Some find it erotic to be doing so while the spouse is unaware. Some feel the need, but feel guilty. Some feel that what you don't know, won't hurt you.
Many find it awkward to tell their spouse they are perhaps unsatisfied or feel the need. There is a certain amount of security in a marriage, and many women's first thoughts might be does he want to offer this, so that he could then play with other women? Some don't want to address it because they don't want to risk disrupting, or losing, what otherwise may be an arrangement they find security in.
If a man or woman feels the need, its best to discuss and agree to whats acceptable and what's out of bounds. Its great if the feeling is mutual. Many won't want to bring the topic up out of fear that it will end their relationship (...as if cheating wouldn't when finally caught.)