Once you have sex with your BF, can you still tell him no? Of course!
At some point, every virgin faces the big "will you, won't you?" sex question, but once you've gone all the way, does the question go away? No.
Once you've had sex with a guy, it does not mean you have to do it every time you see him -- in fact, telling him you want to wait before the second time is a good way to find out if he's really into you. Any guy who's worth your time will be more interested in getting into your heart than your pants. If sex is the most important thing in your relationship, perhaps it's not the best match for you. The "virgin decision" is not the only decision you make when it comes to sex -- it's a decision you make each time you do or don't.
Then there are times when you've already said yes in the heat of the moment (or in response to pressure) -- but you change your mind. Is it still okay to say no when you're in the middle of yes? Yes.
Your body is yours and you can deny access at any time. No matter how mad he might get, if you're no longer into it, it's your right to end it. But, try to avoid it, indecisiveness comes off as game playing.
Bottom line: You are never obligated to be intimate with your guy, even if you've already slept with him. And you can always change your mind, no matter how far beyond kissing the makeout session has gone.
Saying no is always okay. And if you do say yes, make sure you play safe.
