If you consider yourself progressive, it can come as a big shock to be called out as a bigot or any number of other names (racist, misogynistic, homophobic, transphobic, ableist, etc) or to see someone you thought was an ally be called any of these names. Being an ally means a lot more than just thinking you are one, here are some simple rules to follow:
Always listen when someone in a minority you claim to support is talking to you. Do not shush them, belittle them or tell them their experiences are wrong. Never tell them to calm down. They have a right to be angry (and angry with you), even if it is not directly your fault. Listen.
Accept that even if you think you are on the right side, you may have beliefs that are wrong or need to be challenged and you still have the capacity to offend people. You might get called names, your feelings might get hurt. Try to understand why this has happened instead of just saying "No, I'm not" and getting defensive.
If you see something, say something. Not saying something when you see someone being sexist, homophobic, etc is compliance with their incorrect beliefs. You do not have to educate them, but tell them that you disapprove.
(Have you heard "Your Racist Friend" by They Might Be Giants? Basically that.)
There is a lot more to being an ally, but following these three rules are the right first step.
(Unbeknownst to me, xoJane posted After #SolidarityIsForWhiteWomen: So You Want To Be An Ally, Now What? this morning which covers a lot of the same ground. Go read it!)