I'm going to break up with my girlfriend of on-off 3 years because she wants to wait until marriage to have sex
Is there anything wrong with not wanting to have sex before marriage?
She's a religious person and she thinks sex before marriage is an immoral thing.
If you think you really love her, you should respect her belief and not force her for the sex. If you can't wait untl your marriage instead of continuing your relationship, you should break up with her.
I can't wait that long.
Then I guess you need to break things off with her if you can't respect her decision.