BeR, it would be very sad for people unable to have children, to be denied the comfort and support of a partner. You are quite right that some are born infertile, they wouldn't necessarily know until they try but some do know, eg if a woman is born with something malformed internally they probably would have found out with routine gynaecological examination or maybe if they went to the doctor for contraceptive advice! Likewise, there are men who always have infertile spermatazoa. These are normal healthy people who can love like anyone else.
If two people are infertile they have another purpose in life, not enough is made of that in our society nowadays but there are endless possibilities for people without children. Not just taking on someone else's children, which some do, but pursuing a career path that changes the lives of others.
Gonnagle makes a good point about there being so much pain in the world, two people of the same sex finding happiness together is a very small thing in the general scheme (my paraphrase), love and commitment are to be celebrated. Homosexuals are a small percentage of the population, that's just how it is, and don't affect the rest of us. I don't know why people are overly concerned, most of us are not gay and thank goodness nowadays gays are able to be happy, openly, without fear of the law coming down like a ton of bricks. That must have been terrible, something we can hardly imagine!