"Homosexuals want to commit their lives to one another and raise children."
As soon as the idea of homo marriage is completely accepted by society, they'll lose interest in it. Hedonism is their bag. Behaving in a 'normal' manner doesn't do it for them.
"Marriage" protects the children."
If it is about what is best for children, a stable family with a daddy and a mommy is best. Two mommies or two daddies leaves the kid missing an important role model. (And no, Uncle Fred can't really fill in - the mommies can only fool themselves into believing he can.)
It isn't about "stability". It is about what happens if there are children and this committed relationship dissolves. How will the children's interests be protected? How will it be decided who has custody? What about inheritance in the case of the death of both parents? Yes, part of it is having society accept them as a married couple, but more importantly are the legal protections that need to be afforded to them.