I think your confusion arises from mixing up "sanctioning" and "accepting". Those are very different concepts. Most churches (well, all in reality) accept sin in their followers (as they see sin as a simple fact of human existence, to be fought against, but unavoidable), and it is a central tenet of most churches that a sinner can be forgiven and return to the fold if he or she regrets their actions and seek forgiveness.

It is something else entirely to sanction sin. In a church where gay sex is considered a sin, a marriage ceremony would do just that. Such a ritual would basically say that (depending on the church) God or the church bless the sin and dispense with the injunction to avoid sinning. Rather than a sin and then remorse, it would be premeditated sin, a carte blanche. I do not know how else to describe it. This shouldn't be hard to understand.