And yet thou wilt, for I, being pent in theePerforce am thine, and all that is in me. (13-14)
This is where the speaker says his mistress is never going to stop torturing him because he's "pent" inside of her. So, "pent in thee" literally means "imprisoned in you." On one level, he's continuing a metaphor from the previous four lines, where he said his heart is being held prisoner by his mistress. But here, our speaker is also making a pretty blatant reference to sex, or what it feels like to be physically and literally "pent" inside of her while they're getting busy. And, yeah, he's also comparing her vagina to a jail cell—the implication being that he is totally powerless (like a prisoner) when he's with her, especially as he climaxes and gives her "all that is in" him.