considering that heteroloveuality is predominant in our world, i'd say that it's a safe bet to assume that a girl is into guys
Current estimates range around 5-10% of the population are gay.
If 1 in 10 are likely to be gay that is quite a sizable amount of gay people you will encounter. Enough to make it illogical to assume everyone you meet is straight.
it DOES bother me when they talk like melodramatic 15 year-old girls though
Does it only annoy you if gay people talk like that, or anyone?
Regardless of whether you mostly encounter gays who speak that way, is it the speaking that is annoying or them being gay?