Sadly, many people say yes to sex because they don't know how to say no, or aren't comfortable doing so. You have every right to control what happens to your body. You are in charge of what happens to you. No guy or girl has a right to force you to do anything. You have a right to decide what you are comfortable with, and limit yourself to that.
Saying no is easiest if you don't get involved with people you have to refuse. If you take time to get acquainted first, you'll know more about each other, be closer emotionally, and deal with sexual pressure much easier. Of course, it's still probably best to avoid prolonged periods alone together-- if parents are home, just not in the room, no problem, but an empty house is an awful lot of temptation...
However, if you've taken time to really get to know each other, and respect each other, awesome. Enjoy your relationship! Too often we focus on getting things in life, forgetting to enjoy what we already have. (Hey, here's another reason not to have sex: many people think by engaging in sexual activity, sexual desire will go away. While short-term that may be true, long-term these people end up requiring increasing amounts of activity to satisfy them, and often feel even more sexual desire than before.)
Avoiding possible sexual situations with casual dating is a bit trickier, but it's still pretty easy: until you really know him or her, don't be alone together (in a car, a deserted room, or especially his house.) If you find yourself in an uncomfortable situation, you have every right to leave. No one has a right to force himself (or herself) on you, and you don't "owe" anyone anything, no matter what he or she has done for you or what he or she says. You have no business dealing with someone who thinks otherwise.
Clearly state your limits calmly but firmly. If he or she won't stop, do what you can to escape. Once you get away, break up with him (her). He or she doesn't share your beliefs about the preciousness of sex; this is unacceptable. He/she won't change. If he/she really respected you, he/she wouldn't have done it in the first place. There is someone out there who can appreciate you for who you are. Don't endanger yourself with someone who can't.