A male prostitute is a gigolo. A young, gay male prostitute is a rent boy.
The "Chelsea rentboys" chant comes from a newspaper headline back in the early 80s where a gay MP was paying for sex with a young man in Chelsea. The healine was something like "MP found with Chelsea Rentboy".
The CPS says it's homophobic, Kick It Out says it's homophobic, Stonewall says it's homophobic and the official LGBT Man United Supporters group says it's homophobic.
It's got nothing to do with money or selling our soul, I've been called it since I started regularly going to games in the early 90s.