It's anathema. Marriage has a revered and hallowed place in Mormon theology. It is between a man and a woman, and has no other definition. Active homosexuality is a significant sin.
No practicing Mormon would advocate homosexuality. No practicing Mormon would advocate changing the definition of "marriage" from what God intended it to be.
Mitt's doing so while claiming to be devout demonstrates rank hypocrisy.
Mitt Romney is more than a “devout” Mormon, he is a very powerful figure in Mormonism, and has held very high religious office.
Romney was a stake President over the other Bishops in his district, and he received the blessing to run for President in a personal meeting with the Prophet himself.
Romney’s most aggressive promotion of abortion and the homosexual agenda, was during and immediately upon leaving his 9 year position as stake president in 1994.