it does not mean penis,
reason one, it is not a muscle (blood is pumped into it when you get a hard on)
reason two, when a person means love people generally say it comes from the heart (or thats where the term comes from "oh i love you with all of my heart")
there is a song on american pie called "love muscle" it is about emotions and feeling not about cock/sex
seriously it is not penis.
tom; to bad dude, you looked great together
mark; yeah true, ...... my love muscle hurts
person; i love you with all my love muscle