Hmm...a lot of the time, yes...but it's not always the case.
When I started my job, years ago, one of the guys in Admin came down to take me through some systems stuff. He was with me for about an hour a day, all week.
I was convinced that he fancied me. I just got that feeling...lots of eye contact, unnecessary arm brushes, showed a lot of interest in me generally, quite flirty, asked if I was single (which I technically was)...he was just completely giving off that vibe. I fully expected to have to let him down gently at some point because i'd not long started seeing dh so wasn't interested (although we got on brilliantly and he was ever so nice to look at
)
Anyway, after that week, I was out on the Saturday night with dh and we bumped into him. On a date. With a man 
I would never in a million years have said he was gay. I don't think anything before or since has shocked me so much. But I have since discovered (I still work with him, 12 years later!) that he is definitely gay, very gay, and not bi at all - so it's 100% certain that he didn't fancy me!