Good question. It is not the only serious sin that people care about when considering where to place a child. But when an organization is Catholic Charities and homosexuality is considered a mortal sin by Catholic Dogma, then for us, it is one of many serious sins to acknowledge and comprehend.
Homosexuality is a mortal sin against Catholic Dogma, just like adultery is and lying, cheating, stealing...
If a couple has a gambling problem and/or has an acholicism addiction and/or drug addiction/problem, as a Catholic adoption agency, in fair conscious, they could not allow this couple to adopt a child. The couple would first need to confess, repent and amend their lives. The problem with people who participate in the sin of homosexuality, is that they do not realize or comprehend, nor probably care, that they are in sin and need to confess, repent and amend their lives and so out of faithfulness to Catholic Dogma, Catholic Charities cannot allow homosexual couples to adopt.
I realize that this seems harsh and hurtful and I'm sorry and sadden for the pain and suffering that some have gone through due to the "system". I'm not sure what the answer is, but I do know that babies need love and a stable home with a mom and dad. Even in a separated/divorced home, a child still will have a mom and dad. In an same sex home, they truly only have either two moms or two dads. This is how I view it.