"Actually I am planning to getting married to that girl, just not right now, I mean I'm 22, maybe in 5 years or so therefore I can't understand her position."
Too bad she has compromised herself and participated in oral sex with you. I think that may be what gives you the impression that going further is okay because you "plan" to marry her in the future.
Plans change and sometimes more radically then we can ever conceive. Please be a real man and respect her wishes and stop pushing the subject. If you truly love and care for her well-being you will respect her decision. If you just can't wait then obviously you aren't meant for each other so feel free to go elsewhere.