I once heard it was a biological/evolutionary trait that men tend to be givers and women takers in relationships in all respects except that of sex, where it is inverted.
It takes a lot of resources to settle down and have a baby. All the time spent bedridden, the increased consumption of food, the potential risk of life or health in having a baby, and lastly the time spent raising the child, all add up to a large cost to (usually) the mother. It's tough to be productive in a hunter-gatherer society while you're pregnant. Thus, in ancient times, men tended to provide the resources and women tended to provide the babies, and the human race evolved biological tendencies to enforce that relationship which persist to the modern day.
As a by product, it was argued that men are more likely to feel comfortable with treating relationships as means to have sex, and women are more likely to feel comfortable with treating relationships as a means to get everything else.
Another corollary of this line of thinking is that women unconsciously tend to feel the need to wait until she feels a man is committed before she risks having a baby, even in today's age will all the potential for birth control. Thus, the men tend to have to entreat the women in relationships with signs that they will be committed fathers, while for women the sign that she's into you is her willingness to have sex. This kinda means the men feel the need to perform the initial "bone throwing" and most of the subsequent "bone throwing" until the relationship has been developing long enough for her to be comfortable having sex with you, which is something she's primed to do only when she feels the relationship will be long-term; meanwhile, the women feel no such need throwing any other bone to you other than sex.
This theory can explain the tendency to call women who sleep around sluts, and men who sleep around studs; for the women, she's throwing her biological "commodity" around willy-nilly, while the man is such an amazing guy that girls are throwing their commodities at him left-right-and-center. People are reacting to a biological tendency in them which considers a woman's fertility to be her most coveted resource. It can also explain the larger proportion and cultural acceptance of female homosexuality compared to male homosexuality. Men look to relationships for babies, which they can't get from other men; women look to relationships for resources, which they can get from other women. Similarly, the idea that men feel attracted to women of particular ages but women don't care as much about about age is related to female fertility. Another example is friendzoning.
Whether our brains actually do have the biological tendencies outlined above is open for empirical validation, but I think it makes sense in the context of history and given the examples outlined above. Now I'm not saying that things ought to be this way, or can only be this way. The recent rise of "cougar women", for example, represents a bucking of these biological tendencies. What I am saying is that these tendencies can explain a lot of weird or seemingly illogical facets of relationships in our modern world.
