Flirting, like a butterfly, is rather hard to put ones hand on, and once one has, it suddenly seems less beautiful (and sometimes leaves powdery marks).
But seriously, flirting is very hard to define not only because “some people find anything short of intercourse non-sexual”, but because if flirting is done properly, the accomplices define the rules as they go along. Much of flirting is indeed reserved for wooing situations, and this often goes along pretty well defined rules, because people like to know where they stand while negotiating a contract. On the other hand, some of the best flirting is done as a means of friendly communication, as if saying “I find you attractive, and something might have worked out if the situation weren’t as it is”. This type of flirting is fun, and as long as both sides know what it’s about I have no trouble flirting like this with gay or married people, whom I’d never sleep with.