This is kind of an ambiguous question because sexuality, as we now know, is defined by a continuum rather than a dichotomy (or trichotomy), as is the usual popular representation. I would dare to say that the vast majority of situations in which sexual orientation is assumed is in discussion of the attractiveness of someone of the opposite sex.
It turns out that just like most straight people are not 0's on the Kinsey scale, most gay people are not 6's. For this purpose, even most gay people could respond normally to a conversation about an opposite-sexed person's attractiveness. The number of people who score 6 on the Kinsey scale is small enough that assuming a person is attracted to some extent to the opposite sex is a reasonable assumption for most basic interactions. For those people who are a 6, they should recognize that the assumption is made because it is generally not socially acceptable to explicitly inquire as to another person's sexual preferences, especially with strangers. With friends, you shouldn't be offended if you are still in the closet, as you're outright pretending to be straight, and if you come out of the closet then those friends should know anyway.
For other situations the same general principles apply. In general, gay people are just like straight people, except they like their own sex instead of the opposite, so for most, even social mannerisms are identical. The "flamboyant gay" is actually a pretty rare subtype- I've only ever seen one really flamboyant person in 4 years at a liberal university.