A switch hitter is a person who is sexually flexible. The phrase is a synonym for bisexual.
This colloquial phrase comes from the baseball term for a batter who bats right-handed against left-handed pitchers and bats left-handed against right-handed pitchers. Just like these athletes, switch hitters also demonstrate a type of flexibility. Similarly, as the athletic switch hitters don’t seem to have a batting preference, switch hitters don’t seem to prefer one gender over another.