Is this joke offensive?

My sister was in a play yesterday about a comedian. The comedian told a particular gay joke which my sister’s lesbian friend reacted to very badly (read angry). I don’t think the joke was offensive, but I may be missing something due to my privilege or something. The joke in question?

Riots!

While I was off watching a movie about people who had plenty of cause to riot, an actual riot occurred in Singapore where there isn’t really any cause for one. Apparently some people do not know who the enemy is because somehow they think attacking ambulances is going to solve problems.