I don’t see how straights invented “gay marriage”; homosexuality is an obvious mental illness with no rational explanation for the bizarre behavior. Straights invented contraception thousands of years after Spartans (or Sodom & Gmorrah) did the “gay thing”.
It's not true that straights invented contraception thousands of years after Spartans (or Sodom & Gmorrah) did the gay thing. The first recorded act of contraception was in Genesis 38, an episode which originated the term "Onanism," which means turning away from the natural procreative function of sex.
Contraception is condemned in the Didache, an early Christian document which was written probably even before the Gosepl of John, in the late first century.(Link.)
Natural marriage is man/woman, creates a marital bond, and typically results in childbearing if the people are physically healthy. Contraception, on the other hand, facilitates promiscuity and adultery, undermines marriage, and deliberately avoids a normal union and its natural results.
That vitiates marriage to the point where is is disappearing among straight people (half of the children born to women under 30 in 2011 were born out of wedlock). It also tends to redefine marriage as something which essentially "about" the gratification of adult desires, rather than something that is essentially "about" creating a family by begetting and raising children. That redefinition suits the ideology of gay marriage to a T.