There going to be a negative effect on adoption, but considering that some states don't allow same-sex couples to adopt children, that point is sometimes moot. I'm also inclined to say that whatever downtick that happens with the 4% of same-sex pairings really could be offset by mixed-sex pairings (96%ers) picking up the slack, especially since mixed-sex pairings are the ones generally responsible for the need for adoption in the first place.
This discovery enables same-pairs to enjoy a major option available to mixed-sex pairs, so I consider this to be good news.