Why do cons think gay people can't be good parents?

I hear this argument from the right that a hetero couple is preferable to a gay people. Really? What if the gay couple are two harvard graduates with six figure incomes who are also philanthropists? What if the hetero couple lives in a trailer park and the husband is an alcoholic wifebeater? So this argument sounds spurious to me. Why don't people look at the individuals rather than the ual orientation? People aren't just 'gay', they also are human beings. Are gays good parents? I don't know. Why don't we take it on a case-by-case basis? Would Lance B make a good parent? (I think yes) Would Ellen Degeneres make a good parent? (I think yes) Would Elton John make a good parent? (probably not) It's not enough to say, "Well this couple is gay and this couple is straight so obviously hetero couple wins." Personally, I think Ryan Seacrest and Cheney's daughter would make good parents whereas Ted Haggard and preacher Eddie Long would not.