Strictly speaking, sinfulness happens when you fall short of the good beings that God has made you to be. The root of it is self-centeredness. Whenever we make selfish decisions, we are tending toward sin. Whenever we act in the best interests of others (selflessness), then we are tending toward Godliness. Does this make sense? You will also find that when you tend to be selfless...your experience will naturally be far more pleasurable than the experiences produced out of self-centeredness.
Masturbation: I suppose that at its core, this is a kind of selfish endeavor. But lets be careful not to exaggerate the case. It is obviously quite natural to arrive at an event like this by simply exploring your own sexuality. In that sense I would say it is clearly not sinful. However, when it becomes an obsession, and you find that you are constantly wanting for what you do not have...then you will find that when you lose yourself in this obsession, it could lead to quite negative outcomes. It may cause you to obsess about your own pleasure during the real sexual act instead of focussing on the pleasure of your partner. IOW, it may disrupt the sexual pleasure that a healthy relationship would bring.
Fornication: This one is tougher to get a clear handle on. But you need to consider this as your guide: do you seek sex merely to satisfy your own desire (a selfish motive) or do you seek it because it leads to the betterment of your partner(s)? Keep in mind that 'betterment' does not imply making them happy for a while. It means this: are they are made better people by engaging in this kind of relationship?
My experience is this: casual sex is generally a selfish endeavor...and in the long run it does not bear good fruit. Lets not quibble over 'right' and 'wrong' for two consenting adults. Simply try to take an objective look at the long term benefits or detriments to such activity. Can you identify any benefits? If not, then perhaps its worth carefully considering such activity before engaging in it hap-hazardly.