Rent Boys are, as the term implies, usually young boys who live on the streets and attract customers by hanging around in known areas - streets, cafes, toilets etc.
A male prostitute can be a man who has sex with men for money, but as Gok Wan states, the term can also be used for men who have sex with women for money.
I don't think rent boy are always gay. They are often straight young guys making money from older homosexual men. Piccadilly Circus tube station used to be notorious as a pick up area for rent boys and their customers.
Sadly I am not ignorant of rent boys and many I have met loathe the homosexual act and the men that rent them.
They are heterosexuals making money, usually for drugs. There aren't many women willing to pick up young men and pay them for quick sex - so they sell themselves to men. This doesn't make them gay.
Of course, many rent boys are homosexuals. But I am neither fatuous nor ignorant about this subject.