People forget all the reasons gay marriage had to be fought for; no matter how solid a relationship is, the external privileges of having that status recognized matter.

(Also, few of the posters in this subthread are looking at the woman's side of the game theory)

Where I come from, public health insurance is a given, so no need to marry to obtain that basic "right". Marrying to fascilitate immigration is not unheard of, however, most cases I know are more about economic incentives then actual love. I can not speak about lowering tax burden, the system I am living in is apparently too different from yours.

The father only gets custody if the mother is proovably insane or basically unable to provide for herself. In all other cases, she is treated with extra priviledge.