You should have put this in the dating forum. =/
But anyway, if you're not ready in every aspect, then don't let the dirty sod anywhere near you >
If he doesn't care about you then it could unfold like either of these:
Scenario A: You'll do the business and he may or may not leave you because he feels satisfied. If he doesn't leave, then the amount of respect he has for you will probably at least fall a bit.
Scenario B: You'll tell him where to stuff his condom and he'll leave you because he's disappointed he didn't get laid.
If he DOES have feelings for you, then if you refuse, he'll probably not mind and stay together with you, and if you do, he'll be enchanted bye you. But like I said earlier, if you're not ready, it's not a great idea.