Even for those in committed relationships, waiting for sex is a totally viable option. It doesn't have to be about religion either--it is simply a decision about boundaries undertaken by partners of any relationship status or sexual orientation.
We prefer to use the term 'waiting for sex,' since the word "abstinence" has such a negative connotation. "Abstinence," however, is scientifically defined as abstaining from sexual activity including oral, vaginal and anal sex or genital touching. People who decide to wait for sex often create their own definition of what waiting means for them.
The frequency of, or your decision to or not to have sex is not a binding contract; you can always change your mind! Likewise, even if you have had sex before, there is nothing wrong or strange about taking a step back, or taking a break from sex entirely until you decide you are ready to start again.
According to the American College Health Association in 2009, 30% of current college students reported NEVER having oral or vaginal intercourse. Additionally, 30% reported having had oral sex, and 20% reported having had vaginal intercourse, but not in the 30 days prior. So when you hear people say "Everyone's doing it," that's probably not the case.
Remember: you can contract STIs from ANY sex act. Be prepared. Use protection.