In this verse, Paul is telling us that sex within marriage is a good thing, but also that there are times when it is acceptable to refrain. Importantly, he tells us that a reason to abstain might be to concentrate on prayer. By saying this, Paul is setting up the idea that sex does in some way interfere with our connection to God. If we are to really devote ourselves to prayer, it may be a good idea to abstain from sex for a while. So, while Paul does clearly say that sex within marriage is a good thing, in this verse he implies here that it can also get in the way of our relationship with God.