- Do not repeatedly condemn or distance themselves from the bigotry ("He has a right to believe what he wants," "he's not like other rabbis," "I do not like you using that language to condemn him"), or
- Feel that atheists or free-thinkers putting up signs drive away the open-minded ("Why be offensive like him and get in their face instead of reasoning it out? After all, Jesus said...golden rule...blah blah blah")
And if the moderate/liberal are offended enough by a sign to remain align with the bigoted, they are not exactly going to support your cause anyway. But some of them will be moved enough to see that people truly should resent this bigotry.
I give you The Fucking Homophobe Rabbi Yehuda Levin:
Go shake your male member in a motorcycle wheel, rabbi!