Look. We have no idea if our speaker is actually "in love" with his mistress, or if he just really likes hooking up with her, despite the fact that she's also romantically involved with his BFF. (Check out "Themes: Sex" if you want to know more about the hooking up part.) Here's what we do know: the speaker of Sonnet 133 uses a boatload of clichés from popular love poetry to describe the emotional pain of being in a love triangle. (His heart's been "wounded," he feels like a "prisoner" and a "slave" to his mistress, etc., etc., etc.) We also know that, even though our speaker is ticked off at his two-timing mistress, he absolutely loves and adores his friend. (Yeah, the same dude who's been getting busy with our speaker's girlfriend.) We talk more about that in "Themes: Friendship." As far as love goes, though, this poem sketches out one bizarre triangle.
When the speaker uses a bunch of cheesy clichés to describe his pain, it makes us wonder if he's really all that heartbroken. Who knows? Maybe he's just being a drama queen and a poetic show-off.
Even though the speaker uses a boatload of poetic love clichés, they're super-effective because they really do give us a sense of the speaker's emotional pain and suffering.