I don't mean to play devil's advocate here, and I also do not agree that the only appropriate setting for a sexual relationship is marriage. But I do feel like there are good general standards that can be followed.
One must keep in mind that even practing safe sex leaves the possibility of the girl getting pregnant, which if both parties are not at least nominally prepared for it will cause a lot of problems. Either the burden of a child before one is ready or the emotional baggage of having aborted one's child, a decision which can haunt one decades later. So I would say the first standard to keep in mind is maturity.
Secondly I recommend that love be found amongst the couple, for one never knows which partner one may have to raise a child with.
These are personal standards however not ones I would push on anyone else. It is entirely possible that by advocating its strict stance on such matters the WTBTS is merely trying to avoid any lapses at all, however no matter what their intention no number of rules and guidelines can fully counter human nature, and in the end it seems more likely to cause one to have an unhealthy focus on sex and then have feelings of guilt over it.
