This is irrelevant. If society wishes to reward people for having children, it gives people benefits in exchange for having them, as indeed it does in the case of child tax credits, maternity leave, tax breaks for college tuition, and other things of this nature. These benefits are specifically directed at the children; they’re given to help with the costs of raising them. But these are not the benefits gay couples wish to receive (unless they also adopt children, which is a separate issue); rather, they say their partnerships should receive precisely those specific benefits that are given for the <i>formation</i> of heterosexual partnerships, unrelated to the process of having and raising children. Saying that marriage benefits are a “reward” for something that married couples haven’t done yet, are not obligated to do, and may have no plans of doing, on the grounds that they are likely to do it at some point in the future anyway, is like saying that finishing high school is the same thing as finishing college, because people who do the former are more likely to do the latter than people who don’t.
This also has no bearing on the point. The point is, a benefit such as the ability to visit one’s partner in the hospital has no relation to whether one has or wants children or not, and thus should not be given on that basis. It is a personal, private affair that concerns only the individuals involved. It is absolutely arbitrary to make it contingent on the ability to produce children. You might as well only give it out to couples who meet a certain SAT requirement, or who can do a lot of push-ups.
Well, then, why stop there? Why not distinguish between different <i>kinds</i> of heterosexual marriages (between people of various races, religions, economic groups, academic aptitudes, medical histories, and so forth), on the basis of these vaguely defined quantifiers that determine “productiveness”? Why not assign benefits to these various groups on a gradated scale, based on how “productive” we determine them to be according to our arbitrary standard? If society is entitled to not recognize, or not confer benefits on homosexual partnerships because it deems them to be “not productive,” it follows that there’s no reason why it wouldn’t be entitled to do the same for any other group. Unless, of course, society thinks that homosexuality in and of itself is the problem, and not some notion of “productiveness.”