There is nothing wrong with thinking about sex when you are single. You can have plenty of sex when you are single, and the fantasies that can arise can be amazing. The unhealthy thing is to confuse sex with love. Lust is a very powerful (and wonderful) feeling, but it is not love. Don't have sex with someone, thinking it will make them care about you. Casual encounters may not be for you, if you cannot separate the physical from the emotional. If you can separate, go out and have (safe)sex and stay single!(There are some that would say singles have more sex then married couples, hahaha). Otherwise, find another outlet for your sexual energy (writing, art, reading erotica, porn, whatever). Whatever you do, don't try to pack your sexuality up and hide it aware somewhere, that could be really unhealthy.
Sorry, I didn't start out intending to sound like an advice column