I'm with engineer - that's a ridiculous, self-serving reason.
You had sex with one girl before you were a couple VS 'hey I want to have sex with some other guy while I'm with you'
How is that in any way right, equitable, or even sane?
You can't even handle her a past foreplay of hers - how on earth do you think you can handle this? Stand up for your own relationship needs. You won't respect yourself otherwise. And neither will she.
P.S. there are plenty of wonderful people, who aren't compatible with each other in a relationship...that doesn't stop either party from being wonderful. You may want to consider this.