“By allowing the service, he accepts the sin of the homosexual.”
Not at all.
We all have sinned and fallen short of the glory of God. Otherwise, why would we need a Saviour?
Are you so confident that the young man has committed an unpardonable, unforgivable sin in the eyes of the Lord? You know for a fact that this sinner cannot be and has not been forgiven for his choices?
The Preacher will answer for his own decisions, as will we all, whether or not our Christianity is grounded in the Word or not.