Fundamentally, all things should be mutual. Whether you consider yourself submissive, dominant or nothing at all; where sex is concerned you should both get something out of it. Looking at it from a cold-hearted perspective, sex still needs to provide something. Perhaps you both hate sex, but want a child - that makes it worth it. A prostitute gets money; the customer leaves happy. Even when pleasure isn't involved, if there isn't a benefit of sorts (a sense of security etc) - something's wrong. For this example, assume physical pleasure is the only important factor for the people concerned. Let's say your partner comes home, demanding sex. If you don't feel like sex but submit to that demand, it's basically rape. Or, at the very least - the relationship is one-sided. I'm repeating myself now, but both parties should benefit from it in some way.