Your example only works when comparing two perfect families, i would probably concede that the perfect man+woman family, would be able to offer a more rounded upringing than the perfect man+man or woman+woman family.
But just because a marriage is man+woman, doesn't mean those individuals are going to do well, in fact most humans are flawed and as such its ludicrous to rank family's on something as pointless as gender. You should rank parents on criminal history, social network, income and leisure time available. Because those are the things that'll affect the child the most during the crucial younger years.
If your family, regardless of its composition, fullfills the requirements for adoption (the non bigotted requirements) they should have equal access as any other family, but still ranked on the same terms as "normal" families.
1: Marriage has become more than a religious thing in society. There is alot of legal bindings and social aspects to it. You don't even have to be a christian to get married in most countries. And i still dont think two straight junkies will make better parents than acomplished homosexuals. Gender and family composition shouldn't play a role here, focus on the human aspects of the family environment, You're not here to judge, but to make a decision on wether they are capable of taking care of a child or not.
2: this just serves to make adoption/insemination even harder / impossible. Since nobody in their right mind will donate if they can get stuck with child support and whatever. Young people, (students) tend to be the most virile and best contributors for sperm donation, and they dont have the means to support a child, yet they have sperm that could help many families looking to make one.. Following your example only creepy 30+ year olds who couldn't find a mate would even risk putting their sperm out there, hoping to get caught in some random womans game, rather than for the monetary compensation students get enticed by.
TLDR: You got the wrong angle, accept that gay love doesn't hurt anyone more than straight love does. And that nobody is perfect so gender composition becomes even less of an issue when facing all the things a family could suffer from, gay is fairly harmless, we have more homeless kids than we have "great" adoptive families, so allow "great" homosexual families to provide a home for the young'uns.