Wrestling and only out to two other "straight" dudes. We found out about each other one night we were camping together. As it turned out, we were drinking Bacardi 151 and I awoke, after passing out, getting jacked off. I couldn't get my drawers down fast!
Tennis, but amateur competition outside the high school, which didn't have its own courts or a formal program. I wasn't out in high school at all, though my frequent boners in the gym class gang showers might have spoken for me.
i rowed crew (4s) in highschool. and i was on varsity crew (singles + doubles) and swimming (100m backstroke 100m IM) in university. i think people suspected, but i didn't come out until after university.