When Chandler talks to Monica about all the jokes the friends have kept making, he reacts immaturely by trying to pass off their relationship as "casual". Monica gets angry with him and storms out, avoiding him when the two get within talking range.
When Chandler discusses the jokes the friends have been making about their relationship, he acts childishly by trying to portray it as casual. This angers Monica, who leaves in a huff, refusing to engage with him until she has calmed down.