Why does same-sex marriage happen?

Answer 1

When same-sex marriage happens, it does so for the same reasons as any other marriage. Two unmarried persons are (presumably) attracted to each other, love each other and are willing to commit to a legally binding relationship for the purpose of starting their own family unit (with or without children).

When these same circumstances come together in a jurisdiction where the law does not permit same-sex marriage, then it occurs as either a civil union, domestic partnership, or an unrecognized life partnership.

The absence of recognition does not prevent same-sex couples from pairing off and starting a life together, although it does make their lives more difficult in the areas of taxation, inheritance, health care, etc.