The difference is your unwillingness to accept a different opinion.
I, for one, can accept a different opinion, when that opinion is presented for logically sound reasons. I can have the opinion that marijuana is not bad for society but if I simply state that opinion because I like marijuana and can provide you no reason beyond that, then my opinion is worthless.
Your opinion is that homosexual marriage is less valuable than heterosexual marriage because of the natural "cycle of life" and childbirth. I have responded positively to your opinion, seeing a certain beauty in that opinion. That opinion you hold is based upon the opinion that the purpose of marriage is for procreation. If that is the purpose of your marriage, then that is the purpose of your marriage and I have no right to dictate to you what your marriage should mean to you. Where the problems come, however, is that I have a different opinion on the purpose of marriage and that is a lifelong, committed relationship where gender is not an issue. Yet you would claim that my purpose of marriage, because it is different than yours, is less valuable than yours. The unwillingness to accept a different opinion is yours.
But to have children is a virtue. And people should marry to do so.
Then by your sentiment, if a gay man or woman must take a sibling, niece, nephew, grandparent, cousin etc. into their home when they are in need of that home, then the gay man or woman should marry; as it is beneficial for that child to have 2 caretakers with 2 different perspectives on life to share the duties of child rearing. While I do not wholeheartedly accept this opinion, I can see virtue in this opinion; but it is definitely your opinion; so when there are children in the home, by your standards, gays should marry; and now that this child is in the care of that gay couple, that gay marriage now has all the value as heterosexual marriage. These are your own opinions from a different perspective.