Sonnet 137’s speaker is a guy whose lady-friend is massively cheating on him. Massively, folks. So, as you might expect, betrayal is a pretty big theme. The poem doesn’t leave us in any doubt about how the speaker feels about being betrayed. He calls his mistress all kinds of nasty names and describes her with some of the raunchiest metaphors he can think of. So take that, mistress! The speaker even goes so far as to imply that she has slept with everyone in the entire world. Yipes! We don't take him literally at his word, though. That said, it could sure feel that way to our poor speaker, once he realized the truth of his lover’s betrayal.
The speaker is angriest about being betrayed in line 13, because in line 13 he points out that the woman who betrayed him now "owns" him. That really steams his chaps.
Since the speaker admits that he himself is to blame for this betrayal, he will eventually forgive this betrayal. The poor sap.