You were probably taught as a kid to never go along with any kind of sexual pressure and told that setting boundaries can help in these situations. That you should say "no" or walk away from situations that don't feel right to you—no matter how reassuring or pushy the other person is.
Well guess what? Setting boundaries is the golden rule in sex and relationships for the rest of your life. If you don't want to do something, if you feel pressured, or if you think something doesn't feel right, then you should say, "No," loudly and clearly. This will set boundaries for your partner. That includes everything from NOT kissing a guy just because he wants to kiss you to not giving in to sex just because you feel pressured. Tell an adult that this is happening and keep telling until someone listens.
Why should you practice setting boundaries? Because sex, romance, and love are wonderful parts of life, but only if you learn about them at your pace. If you betray yourself by not setting boundaries you may carry around bad feelings for a long time, and you may have trouble feeling good about love and sex in the future.
Other Sources of Pressure
You might think that we only feel "sexual pressure" from someone else. But we also feel pressure from ourselves—from our own desires, our own curiosity, or the excitement of doing something new or of rebelling.
And we feel peer pressure. You may think you're over that, but studies show that what we decide to do or not do sexually is most influenced by what we think our peers are doing. Just be aware of that fact and try very hard to be true to yourself, even if that is not the "cool" thing.
As for the pressure we feel from our own desires and curiosity, those feelings are fine, too, but we can't just act on our desires all the time. Think, and weigh what's right for you in a given situation. And for you, that may include moral and religious considerations. It should definitely include thinking about the consequences of your actions and being responsible for others as well as yourself.
Setting Boundaries for YOU
Decisions about sex and relationships are complicated because humans are complicated. One of the most difficult things in life is trying to understand ourselves. So often, girls decide to go ahead with fooling around or having intercourse instead of setting boundaries. Counselors talk with teenage girls all the time who do stuff to be "popular," to be liked, or hoping to find love. A lot of times these motivations result in disappointment and hurt.
The bottom line is: Be true to who you are by setting boundaries, make decisions that feel right for you, and act out of your own needs—not someone else's.