The five precepts constitute an integrated set - each precept supports the others. Sexual misconduct', in the spirit of the precepts as a job lot, means any sexual conduct involving violence, manipulation or deceit - conduct that therefore leads to suffering and trouble. By contrast good sexual conduct is based on loving kindness, generosity, honesty, and mental and emotional clarity - conduct that has good results. Buddhism's very tough sexual ethic would be complete without the third precept. 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.