The definition of the traditional nuclear family unit, and the inherent roles and responsibilities as we currently know them is very much geared towards man-and-woman relationships.
You could argue that we should change these definitions - but until the, how the majority of people in most countries think of families is very much coloured by the concept of the nuclear family.
Then again, times are a changing - so maybe you'll argue that in the future, we'll do away with the concept of this procreation business, and just clone humans, or have surrogate artificial wombs and we'll just pick our kids off an assembly line fully grown =).
For starters:
* Members of a couple (same-sex or opposite sex) have family obligations to their partner's extended family.
* A surprising number of same-sex couples have biological children from earlier relationships.
* The "traditional nuclear family unit" is not that old a tradition - prior to large-scale industrialization and easy migration, extended families were the rule. It's just one kind of family. Others exist.
