It depends on a country we live:) to my point of view a lesbian couple and a couple of gay men should be stopped from adopting. Our (Russian) legislation decided that such couples must not adopt a child. As for me, I absolutely agree with their statement: unisexual couples will be followed as an example of ordinary family life. We can not allow them to do it:( of course, I understand that these couples won't grow up a gay boy, or a lesbian girl, but I don't want those children admit unisexual relationships as ordinary relationship in the family.
I tried to write a small paragraph, sorry for the absence of arguments:)