I often pass as straight unless I start dropping hints to people. It doesn't normally matter, but there have been many times I've deliberately acted a bit camp in order to implicitly suggest my sexuality (e.g. if a woman seems sexually interested in me). Sometimes I need to be more direct with people, but it can embarrass them so subtler clues are often better. Of course it also a code for other gay (or bi) men to pick up on and that can have its benefits.
You've hit an interesting point about straight pubs and a very exaggerated form of masculinity, which is often quite 'artificial' (or at the very least alcohol induced). A lot of gay bars mark themselves out as being distinctive and different by consciously going the opposite way. But I don't believe gay men are naturally camp, more feminised or more prone to liking Kylie.