The game changes when two people are romantically involved or "semi-involved" (a fascinating phrase I recently heard). Before you start throwing things at your computer, let's go to Scripture.
We are to do this in light of what God has done for us in Christ and in light of Christ's impending return. For God did not call us to be impure, but to lead a holy life.
The argument becomes clearer when we look at some of what the Bible has to say about 1) sex, 2) our relationships with other believers and 3) sexual immorality itself.
As a good initial principle here, we should affirm that sex itself (and sexual activity in general) is not inherently negative or sinful.
If you have any doubts about God's intention to give us sex as a wonderful, pleasurable gift, Song of Songs should put them to rest.
In Song of Songs, God has given us a holy and beautiful picture of a marital sexual relationship, and everyone seems to be having an excellent time.