You shouldn't say rude things to people.
Adam is talking to Joe and notices that the fly of Joe's pants is unzipped.
Adam wants to let Joe know that his fly is unzipped.
Adam quietly tells Joe that his pants are unzipped.
Adam tells Joe that he is gross for having his pants unzipped.
Joe is mildly embarrassed but thankful that Adam told him his pants were unzipped.
Joe's feeling are hurt by Adam's words, and he zips his pants.