in layman's terms, what actually happened to you is you death-gripped your dick into neuropathy territory, so no, zoloft did not directly make your dick go numb. a fairly common side effect of SSRIs is they render men unable to ejaculate/orgasm, either fully unable or make it significantly more difficult so without even noticing, you started masturbating more aggressively to compensate and voila, you lose feeling in it