I'm afraid you won't be able to join the RAF.
You see - I'm the only gay in the Air Force!
Seriously, although the previous advice may seem to be fairly right wing, it is both accurate and sensible. Although many in the RAF couldn't really give a toss whether you are gay or straight (as long as don't chat them up), there are still a reasonable number who would find the situation difficult to deal with, despite compulsory diversity and equality training.
Aircrew tend to be judged on ability, not sex or sexuality. If you are a nice bloke, reasonably good at your job, and don't make it overly obvious that you are gay, most guys on the squadron will accept you for who you are. The RAF (in theory at least) will come down heavily on anyone who tries to make life difficult for you.