Marriage is between most men and women for the main purpose of beginning a family. If it wasn't for the sake of the children, I suspect most men would not even bother to get married. Clearly, men who visit whores, do not consider themselves married to such. So, if sex is the only consideration, there is little physical need for men to marry women.
When a man cares enough for a woman to want to both support her in begetting children and stay with her to raise those children, then that is when a marriage committment becomes very important. In the case of homosexuality, there are no children to be had. The only reason for marriage becomes physological and material.
A government, has little to gain from promotting sexuality; however, there is plenty to be gained from promoting constant relationships that foster parenthood. Homosexuality doesn't foster parenthood. The truth be told is that homosexuality has likely done more to destroy marriages then it has to make a comfortable atmosphere to nurture children.
Husband and wife relationships are far from perfect; however, the promotion of homosexual unions is not going to improve marriage, but confound and confuse it. In the end, it are the children who will suffer and society will unravel faster and further.