A song about liking someone that has a girlfriend?

Does anyone know any songs that relate to really liking/being in love with someone, but you cant be with him because he has a girlfriend, even though you both have feeling for each, but you cant be together because hes in in love with someone else?

or a song about not being able to spend time with someone, because people might find out that something is going off between you and he doesnt wanna be caught cheating?

by the way, its not for me to listen to and imagine im singing it!:L its for my dance class, were doing a play on music:-)

thanks a lot! xxxx