Society is better off when offspring have a family- and that means a mother and a father who stick together and raise the offspring to become productive, educated, responsible adults. Since gays cannot have offspring they should not be allowed to have the advantages that those who "sacrifice" and do marry and stick together and have offspring is the thinking and view for most.
This is changed by allowing adoption by gays who otherwise fit the criteria for responsible adults wanting offspring.
As an adopted child myself I'm glad I was not raised by a gay couple, but I'd have rather been raised by a gay couple than live in foster homes. I could care less if a person is gay, straight or bi as an adult. I rather like gays in that they seem to have a personality trait that appeals to me, and I've been mistaken by straight folks as gay many times. I think if I was raised by a gay couple the mannerisms and habits that most can detect with "gaydar" would have been passed/taught to me- and in the current world we live in it would be a disadvantage. Again, I have no desire to have be anything but straight, but do not care what anyone does sexually with willing partners.
I'm not sure where this leads me to, but I think "marriage" should be for heterosexual couples. Let gays have civil unions with all the same benefits as marriage but do not call it marriage. Married couples without kids should not have the benefits as those married with offspring IMHO.