* Most men strive to achieve high status and thereby get a lot of sex. This drive begins as a teenager and may not ever stop until they drop dead. Sex with only one woman for a lifetime is not a high priority for many men.
* Women have a strong need in their DNA to pass on their genes to their children. But historically this was not easily done without protection and support from a man. Because this need still is in women's DNA, women still place a high priority on finding a man who will stick around to protect and support them and their children.
* Most women want a good sound relationship going before giving sex to the man she selects as the best one to insure survival of her and her children. This drive is even in women who are just dating for fun and not interested in having children.