See if you can tell where it hurts during sex. If it is the skin on the outside of your vagina, down by your bottom, chances are that your vaginal opening is smaller than normal. A doctor can help you fix that. If it hurts inside your vagina during sex, what kind of hurt is it? A burning feeling can mean that you have little cuts or tears inside or that something is irritating your vaginal walls. This could be caused by lubricants, soaps, or lotions on your partner's skin, or if he is just simply too big to fit inside you, or like Dora said, you may be allergic to his semen. If it's a dull pain or an ache way up inside, it could be that he is hitting your cervix. My advise is to use plenty of water-based lubricant, pay attention to your body, and if it is truly bothersome, go see the doctor. Don't worry about the fluid. It's very normal and won't interfere with getting pregnant.
if you're saying a white fluid comes out of your vagina when you orgasm, that happens with some women. it's called female ejaculation. it shouldn't prevent pregnancy. it also shouldn't be painful. if it always is painful, check to see if you are allergic to your partner's semen.