Is it okay to not go to Prom with her?

I am 18 (M) she is 17(f) and it is our senior prom. We have been texting back and forth and she has sent me text's like hey cutie, and we just hung out for about 5 hours a couple days back. I asked her if she was going to prom and she said yea but with another guy. I guess I asked to late :(, but don't want to try and steal her away from the guy she is already going with because I feel that is not fair to him. But is it still okay to possibly pursue a relationship with her such as going out to the movies with her and stuff. Also, she was my only option for prom I mean I probably will not go then. Is it okay to maybe try and ask her if she would want to go with me. Any advice would be great i really am confused on what to do.