The logical, and scientific, fault in the "gays are inferior biologically" myth is this. IF procreation weeds out weakness, why have gays been around since .. thousands of years if not even from the beginning of life? We evolved from something, fi not monkeys .. who btw act gay. Marty's argument that he would not pass down some weak trite, like nearsightedness, is a non-starter because gayness has survived. If it were a weakness, why didn't it go the way of the neaderthals? The answer: gayness is not incompatible with being homo sapiens.
The survival of the species argument is also an obvious non-starter .. since gays survive. And the whole argument is based upon a misunderstanding of sexuality: gay men have been having kids with women for thousands of years, and gay women likewise reproduce. There simply aren't many people who never have sex with the opposite sex, even if their predominantly preferred sex is with their own sex. If someone never procreates outside his/her sex, they won't procreate ... of course people have also been celibate for ...... thousands of years.
One might try go with the argument "well, people needed a lot of kids because so many kids died." But of course, gays never prevented human kind form having enough kids to survive ... in fact, they procreated, see above. And any biologist would tell you that having too few children is not likely to do us in today, while the opposite may be true. So, if we're talking evolution .... maybe gays have an "upper hand." (-:
But you see, the "gays are inferior" folks are NOT TALKNG ABOUT BIOLOGY. Rather they attempt to support their religious myths with misstatements about biology.