In the beginning of the scene Earl and Jenna fight and he grabs her and is about to hit her hard when she tells him she is pregnant. He flips out but then is happy and sings the song - he also gives her a very creepy ultimatum that you'll see. After the song he grabs her and carries her to the bedroom, which she is not happy about, so there's a clearly implied sexual assault as well. Jessie's face when he carries her off is just crushing.