"Reward sex" is what I call the mythical fantastic sex that "pure" married couples are supposed have bestowed upon them by God as promised by the purity movement. It was something that a lot of youth group teens, homeschoolers, and bible college students looked forward to. Through the teachings of purity leaders and the writings of Josh Harris and Elizabeth Elliot, we were indoctrinated with this idea that married sex would definitely be worth the wait. Not just because it was sex, but because God would bless us in that area and because abstinence brought other benefits.
None of these ideas can be found in scripture. There is no real theological foundation for it. But it was very easy to believe. The logic went something like this: God cares about the tiniest details of your life and rewards you for obedience. Sex outside of marriage is a sin, so saving yourself for marriage is obeying God. Therefore, God will reward you for being abstinent until marriage.
I'm not sure how they made the jump from there to the reward being specifically great sex, but I suppose it is the most direct and easy reward to assume. Christians in the purity movement like to describe their marriages like a fairytale, probably because it conveys God's participation and approval in a more impactful way. But because of how common these fairytale stories were, I became convinced that my love life would be a fairytale, too. Fairytales, with their happily-ever-after endings, imply that, because it was fated, the marriage is perfect--including the sex. I think that's one other possible reason we all jumped to the conclusion that God was going to bless us with great sex.
Then there were the arguments that abstinence naturally made sex better. Waiting so long would mean there would be lots of pent-up sexuality to express, and marriage would be this huge release. Men and women who had never experimented with themselves sexually would be easier-pleased by their spouse's touch. Having only one sex partner in your life means you'll never know any better, so you'll be having the best sex of your life by default! (I really despise the last two arguments, because they are based on lowering the bar for good sex, and assume that people can only recognize bad sex by comparing it to other experiences, which is simply not true.)
Here's an example of the type of thinking I was inundated with as a young adult. From the Arndt family blog:
I think sometimes people forget that abstinence isn't giving something up; it's saving something. When you drop coins into a piggy bank, the coins do not disappear! You'll get it all back, only in the form of quite a jackpot!
I would say that one of the biggest misconceptions nowadays is that you're missing out by saving yourself for marriage. On the contrary, I think those who don't set anything apart for their honeymoons are the ones who are missing out. For them there is no awe-inspiring new frontier, no ta-da, no feeling of "wow, here we are!" There's no history and no story. No jackpot stored up or set aside, no fresh treasure to give your new husband or wife. The apples were picked before they were ripe, and now there's little left at harvest time.
By waiting, you get the exact same thing, only with so much more power, passion, and permanence. Every dollar you've deposited you'll get back, with interest! You get a wedding night just like any other couple, only with hearts that have been slowly seasoned by a history of waiting and hoping. You'll have reached the exact same point, only with a legacy of love and longing leading up to it.
What a wonderful feeling it will someday be to realize that you've made it across the finish line.
I, personally, was convinced that any sexual experience I had before marriage, even masturbation, would detract from the intimacy and joy I would get when I one day married. I thought this was a fact of nature, that previous sexual experiences took away enjoyment from any relationship. I was led to believe this by shame-based purity teachers who were married and should have known better.
This has always frustrated me, how the adults could have spread and encouraged these false truths, when they themselves had experienced married sex. I assume some of them thought the end justified the means. Some had probably convinced themselves it was true, and that they were either living the dream or suffering the consequences of their own sexual choices.
But as most people realize when they begin a sexual relationship for the first time, sex is not so black and white. Abstaining doesn't make anything easier or better; in fact, it almost ensures you will get off to a rocky start. Sex works like anything does in a relationship between two people. It takes familiarity, practice, and communication.
Whenever the question was brought up, "what if the sex is bad?" the automatic response amongst purity proponents was, "we'll have the rest of our lives to make it better." It was the closest thing to practical and realistic they ever got: an admission that sex may take practice to get better. But it simultaneously dismissed the concern of incompatibility, leading youngsters like me to assume that any two people could make it work, and that with time, any sex problem could be solved.
I don't know if being told the truth about the non-existence of reward sex would have changed my mind about purity when I was young. I think it would have been good to dispel the magic of the purity movement, so that even though I still believed that premarital sex was wrong, at least I would have recognized the sacrifice I was making by committing to abstinence, rather than working towards a reward and pretending there were no downsides.