I don't know if I really believe in people just being "sexually incompatible", and that's the end of the story. I think sexual incompatibility has an underlying cause, like one or both partners not actually being attracted to the other, or a lack of communication, or people being uncomfortable with sex or with their body, or just inexperience. Most of these things can be ironed out with time and practise. Yeah, sex isn't generally going to be great the first time, but with mutual love, trust, communication and attraction, I think the majority of people could eventually have good sex.
I don't think waiting to have sex til marriage is really a good thing, and it's definitely not for me, but I would disagree with the idea of a "test drive" being absolutely necessary.
Well a lot of things could make someone sexually incompatible with another. One person could be too closed-minded that the other gets sick of doing the same thing over and over, unwilling to "try new things". People can even be too open-minded for others to feel comfortable with them, and then there are things like differences in sexual appetite that factor in when talking about sexual compatibility. Yeah, I agree that a lot can develop over time as you get to know what the other person likes, but not everyone is as open minded as you and I, for instance and although some people are okay with doing a set position/positions and acts, not everyone is.