I think the influence of both genders on a child can be overstated. I think it has more to do with the approach to parenting than the actual gender, though I can see your point. In this case though, you'd also have to place additional restrictions on single people wishing to adopt as well, as there would be only one gender in that case as well.
An odd (or maybe not) thing, is that I believe it's easier and more accepted for lesbians to raise a child than homosexual males.