marriage law/divorce law changes annually.
Your premise utterly fails on the child support front.
In fact your argument falls full square of the lefts/ABA view of children as a mere acessory of sex.
Marriage is not based on popping orgasms.
Marriage is based on propagating society.
To the best of my recollection of 6th grade biology, marriage isn't required to propagate the species. All it takes is, well, you know. Marriage serves only to extract money out of the donating male after the biological act has been completed and his presence is no longer desired by the female.