They can potentially cause negative effect on an adopted child. Just like we do not opt to have orphans adopted by people with mental disorder there is nothing wrong in blocking orphans being adopted by gay couples.
Just because current day psychiatry has no definite criteria for some mental diseases, it has not been able to define gayism as a mental disorder. It is an unnatural and out of norm behaviour to say the least.
declare homosexuality as a mental disorder and have that stance scientifically supported by medical experts and you may have a point.
at this point there is no credible study that claims that adoption by gayu parents is worse than a child growing up in an orphanage.