I think you run into trouble when you try to define a normal family. I don't really think there is such thing. Every family has its idiosyncracies. That's not to suggest that homosexuality is an idiosyncracy, it's simply that evey family has things that society as a whole might not define as normal. It becomes especially difficult to define that "normal family" when you consider the current rate of divorce. Single parent families are rapidly becoming the norm. I think a family with two loving parents can be a more positive environment than a family with one, especially with all the strife that divorce can cause. I'm not suggesting that all gay couples will stay together forever (we won't really know the stats on that until we also afford them the right to equal marriage under the law, but that's another topic all together) but adopting a child is a very serious undertaking, one that is not likely to be discarded lightly. You could argue the same thing for the birth of a child, but that is certainly not always planned.
And your argument (again) that infertility for a heterosexual couple is nature's way of weeding out the weak falls short as well. Like Ashley said, what about women with endometriosis or women who are brutalized and no longer possess reproductive function? Are we to believe that a woman being raped and beaten to the extent that there is no possibility of saving reproductive function is nature's way of ensuring she can't propogate life?