Where I draw the line on adoption is saying that a mother has an absolute right to refuse any prospective adoptive parents for any reason she sees fit, and a mother asking an agency to search for prospective adoptive parents on her behalf may direct that agency to apply any criteria she sees fit. Such rights overrule any "anti-discrimination" rules. There is nothing wrong with a mother discriminating against anyone whom she feels would not raise her child in a manner she would deem appropriate.
An argument that same-sex couple should not be allowed to adopt a child even if the child's mother thinks they'd do a wonderful job raising her child is apt to be far less persuasive than an argument that a mother who does not want her child to be raised by a same-sex couple should have an absolute right to refuse such adoption. If a mother is asked to specify whether she wishes to allow her child to be adopted by a same-sex couple and she says that she does not, the argument against allowing a same-sex couple to adopt her child is much more compelling than it would be if she had not stated her intentions. Outlawing adoptions by same-sex couples entirely would shift the decision from the mother to the government, thus weakening its moral weight.
Children are not “owned” by their mothers either. I know a mother who sold her baby son to a couple of gay men. Was that OK with you?