(Reuters) – Double Olympic 800 metres champion Caster Semenya offered to show her vagina to athletics officials when she was 18 to prove she was female, the South African middle-distance runner said in an interview with HBO’s Real Sports that will air on Tuesday. Semenya, now 31, burst onto the scene in 2009 when she won the women’s 800m world title by a stunning margin hours after the sport’s world governing body said she would undergo gender verification tests. “They thought I had a dick, probably, Semenya said in the HBO Real Sports interview that will air at 10:00 p.m. ET (0200 GMT). “I to…