There are far more important things to consider when trying to decide whether or not sexual intercourse is "okay" than whether or not the two people are married. The things I think are the most important are if it's safe, if it's for the right reasons, and if it's consensual.
Safety has been a main issue in sexual intercourse for many years, and has developed a massive industry in products to help assure the two (or more I guess) people's safety. Since there is such a large and openly available market for things like condoms, sponges, spermicides, and birth control drugs, it is fairly easy to create a situation that is safe as long as it's planned on. When things get difficult is when two people are together and things begin escalating quickly and unexpectedly. Under those circumstances there is a lot more room to have improper safety measures. A lot of times people will find themselves in these circumstances when they are considering intercourse for the wrong reasons.
In my opinion the reasons behind intercourse should be based on love. This is opposed to using intercourse as an outlet to one's lustful desires. If one or both of the parties involved are in it for lust rather than love, it leads to unhealthy and unpleasant consequences. A more extreme and obvious case of this is that of ****. One person is expressing their lustful desires to an unwilling partner. This is obviously wrong in many ways.
As long as the conditions that I've written about are met, it doesn't really matter much to me whether or not the couple is married. Either way I think if the right conditions are met then sexual intercourse can be a great way to share in one of the most natural pleasantries known to our species. I don't think that social or religious norms have a right to get in the way of that.