Lack of sex or masturbation leads to swollen testes?

Discomfort: Prolonged arousal without ejaculation (and a young man is going to get aroused) will produce discomfort -- up to serious pain -- from too much fluid being produced. The seminal vesicles and prostate are distended, not the testes. You may hear the term "blue balls". You might want to talk with a sensible adult -- maybe even dad -- about your changing mind and body.