To infer that, because all are sinners, none should be parents goes against God's own teaching. God knew every word we would utter before we were born. Yet he told us to "be fruitful and multiply." To have kids. To become parents. Despite the fact that He knew we'd be sinners.
Queers can't do that, because it requires a male and a female to procreate. That morally sound (though sinful) people might thus believe that it is wrong for queers to raise children is in keeping with the social/sexual/parental order established by God.
That's all.
I would agree with you, but not necessarily in the case where the homosexual is the natural parent. It is entirely conceivable to me where, if the father was a serial fornicator, with a revolving door of sleep-over girlfriends, it might be better for the child to be with his lesbian mother, provided she showed more stability and discretion. This is based on my understanding of Scripture, which does not, in my reading, uniquely condemn homosexuality vis-a-vis fornication and adultery.
My goal is not to soften our stance against homosexuality, but to stiffen it against *all* forms of sexual immorality, which are arguably equally destructive.