Is it okay to break up your crushes relationship?

No! He is hers..

No. Not to make you feel bad or anything but if he has a girlfriend then he probably doesn't like you Now if he never talks to her his girlfriend and flirts with you a lot then he might just not be into his girlfriend. And if he breaks up with her then move in on him, bt until then wait it out!

I know it's probably not what you want to hear but I say don't. Do you really want to be with a guy who is easily stolen away? If you succeed, what's to stop some other girl from doing the same thing to you down the line? If he really likes you then him and his girl will probably end up breaking up anyway, so just wait it out. And do you really want to be that girl? The one who breaks up relationships?