It's really there for the sake of emphasis. Sexuality is a very strong energy, the focus of many cravings, vanities and delusions. On the other hand, each of us also has the opposite propensity to act out of friendliness, generosity and wisdom. With moral and meditative training our sex lives can powerfully express this propensity too. Hence the third precept expresses a tough and challenging sexual ethic.
Lets look at the spirit of the precepts as a whole before returning to sexuality.
Freedom is the ultimate promise of Buddhist practice - of the moral training as well as the other two great trainings, in mediation and wisdom. Freedom means letting go of the obsessions, compulsions and inhibitions of our psychological conditioning, and so freeing ourselves to respond appropriately in any and every situation. Often freedom takes the form of restraint, the ability to say no to an habitual or received compulsion, craving, fashion or dependency. Sometimes freedom takes the form of saying yes, a yes that overrides habitual or received fears, prejudices and inhibitions. We can either treat other people and other elements of our environment as objects of our calculation, exploitation and consumption, or we can see other people as we see ourselves.