The substance is an Iranian-manufactured glue that if applied to skin, sticks to it and can only be removed by surgery.
After they glue the anuses of homosexuals, they give them a drink that causes diarrhea. Since the anus is closed, the diarrhea causes death.
Videos of this form of torture are being distributed on mobile cellphones in Iraq.
I once conceived a similar horror film death involving milk and stitches.
edit: a sexuality-neutral death