Pregnancy and the ensuing years of dependency of the child upon the woman, can leave a woman vulnerable physically, emotionally and financially. I think that biologically, pregnancy has no affect on men. There are social constraints that may enforce legal or other obligations, but from a purely biological position the man carries no burden.
Another aspect of reproduction is ensuring that our genes are passed on. Now for a woman this is easy, as she knows who the mother of her child is (test tube babies and hospital mix ups aside). The man, on the other hand, cannot be sure.
Another aspect of reproduction, is either or both parents motivation to provide their offspring with the greatest chance of survival and development. So their offspring become desirable mates for breeding and so on.
How have sex differences in reproductive payoffs affected male and female relationship styles?