"Traditional Marriage" meaning one man, one woman, tends to favor the gender-roles and our society's programing based on what does (or doesn't) hang between the legs.
However this doesn't mean that all men and women actually follow this 'traditional' formula. A lot of people especially in the Westernized nations are slowly breaking the gender-barrier, thus we are seeing things like Stay at home Dads with working moms.
Gender roles are complex, many of the things we identify with the role based on gender is 'made-up' such as the idea that women are somehow 'weaker' than men, or less able to think in terms of politics.
We are programed by what does or does not hang between our legs. I call it genderfication. IF you are born with a penis you are put in a blue blanket, and dad brings you sports equipment. If you are born without the penis you are put in a pink blanket and dad brings you a doll. Boys wear sailor suits, girls wear dresses. Boys get footballs and baseball bats, girls get baby-dolls and easy bake ovens.
These sorts of things defy logic, they assume that a girl will have no interest in sports and assume that a boy won't grow up to be a pastry chef.
Women do tend to be more nurturing and more in touch with their emotional side. This may have something to do with the whole '
I can carry a baby for nine months inside of me' magical ability.
Having this special ability has lead to women developing a different sort of bond with their off spring than men.
Any relationship that works will have a dominate person and a submissive person in every aspect of the relationship. Most relationships the roles switch depending on what part of the relationship we are talking about.
Two leaders and nothing gets done as they are constantly butting heads and arguing over who is to do what.
Two followers and nothing gets done because they are both waiting for the other to decide what to do next.
Gay relationships are not hard and fast on the rules of who leads and who follows. It depends on the people involved and their expectations for their relationship.
However, 'genderfication' also plays a role in gay relationships were we have the 'top' and 'bottom' idea, where one is assumed to play the role of the 'man' (in and out of bed) based on stereotypes of what men are like, and the other plays the 'woman' based on stereotypes of what women are like.