Well, I think the rainbow sash is a dead giveaway of their arrogance, defiance of the church and refusal to repent of their sin and that says a great deal about what is in their hearts. I should think that it would be revealing enough that a man of God might be inclined to deny communion for their own spiritual good. Withholding the sacrament is not meant as a punishment so much as a persuasion to repentance.
"For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord's body." (1 Corin. 11:29)
The Rainbow "is a political symbol. By wearing it they are makes a political statement as surely as if they were wearing a swastika armband in a synagogue.
What about the verse right before?
"Therefore, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of sinning against the body and blood of the Lord."
1 Cor 11:27
I'm pretty certain that being a practicing homosexual and taking Communion is taking it in an "unworthy manner."