Reading ebonmuse's post, Is Sex For Procreation?, I decided to write my own post on this subject and dust off some of my old theories on this whole matter of abstinence.
I have a pet theory about why religions of all stripes love abstinence so much. This is really not based on biology or evolution because I have no background in that, but it's something I came up with because of my background in economics. So I don't have anything to link to or supportive evidence, this is all just my own little theory.
So, the theory is that abstinence could be nothing more than a means for fit males to prevent less fit males from reproducing. If unfit males can't manage the very high entrance costs of marriage, their chances of reproduction becomes lower. Abstinence, then, is more of a reproductive zero-sum game than a moral value. Let's call it a game - the rules are like this: the players all agree that not paying the cost of entry for reproduction (marriage) is cheating, not allowed, in other words it's abstinence. The game, then, is to get married in order to reproduce. And marriage is bidding war where the highest bidder gets the winnings.
There are two ways in which abstinence could work to the advantage of the most fit males. The first way is if everyone's life expectancy is low enough so that an early marriage pays off. In other words, abstinence works extremely well if the game is only played once. I can imagine that in our history, many unmarried males have died early deaths due to occupational hazards, war, starvation, and health problems. Raising the bar on what is required for a man to reproduce, then, creates an additional source of selectivity that benefits the most fit individuals - those least likely of all to succumb to hardships.
The second way it could play out is when abstinence gives a head start to the status quo. In this case, the abstinence game can be played over and over again. So, in this situation, there are two ways for a man to be fit. The first way is to already have all the required status symbols, social standing, wealth, and other resources to win on the first try. The second way is to keep accumulating those resources until the required level of fitness is met. Case in point, women tend to pair up with older men. It is no longer a zero-sum game in the long run, but in the short there is a slight nuance - every time the game is played, the most fit female is taken out of the game by the winner. The status quo always wins the day.
So that's all well and good, but what does any of this have to do with religion? If abstinence benefits the status quo and helps the most fit males find the best mates, how do the dominant males actually get the less dominant ones to voluntarily refrain from reproducing? What better way than with religion? We're all born atheists, but then some of us are taught to believe not in our own ideas, not to think for ourselves, but to accept wholesale the thoughts and desires of people other than ourselves. Abstinence and religion could very well be a match made in heaven. On a related note, saying that abstinence and marriage are moral virtues... now that's just a petty little slap in the face for the losers of this religious dating game.
At this time I want to point out that if this was all that there is to it, I'd actually be fine with that. The most fit males should probably mate with the most fit females - just don't call it a moral virtue, thank you very much. But then again, I think that the rules of the game have changed in the modern world, so much so that none of the things that I just talked about apply anymore. The religions still exist, but the rules of natural selection have changed.
So where do we stand now? I explained before how the more this game is played, the less it becomes a zero sum game, but that the status quo still tends to win every time. And in theory this would still work reasonably well so long as life expectancy isn't too long and the game isn't played too many times. That's because the status quo at any given time becomes a terribly bad way to predict the future. It may even be misleading.
As an example, think of the lifetime earnings curve for college educated versus high school educated people. Or, consider the remaining life expectancy of a 35 year old versus a 25 year old. Now consider the life expectancy of everyone going up at some number like .2 years per year. This abstinence game hinges on something very important - early marriage for women. After all, they are the commodity that men compete for. Now consider a woman marrying someone 10 years older than herself because of expectations she made by looking at the status quo. Given a woman's longer life expectancy, she could end up outliving her husband by decades. It's possible. Had she chosen a younger guy who was less fit at the time, she could have been better off. Anyway, this is just one for-instance.
The point is that the longer we live, the harder it becomes for fit men and women to enforce their dominant roles in the reproduction of our species by the traditional method of abstinence followed by marriage. It just no longer works, even if you believe in religious fairy tales. What works better is for women to deemphasize the status quo when choosing their spouses, to engage in more casual relationships that benefit from contraception, and to just wait longer to make a decision about marriage, if they get married at all. Forget abstinence.