Trieste is not the most welcoming place for LGBT people in Italy (read this wikipedia page for further info on LGBT rights in Italy). This means that although Triestinians are, in general, quite open-minded and there have not been reports either of hate crimes targeting LGBT people or of openly discriminatory attitudes in recent years, there is not a particularly active LGBT scene unlike in some other Italian cities (A couple of gay PhD students have described Trieste as definitely boring even if not a completely dead place).
However there are local branches of at least two major Italian LGBT associations: Arcigay (Circolo arcobaleno - you can find a mailing list there) and CertiDiritti. The former is mostly devoted to promoting social events while the latter works on LGBT rights issues.
Apart from dating websites, another way to get in touch with the "locals" are visiting gay friendly places (like the Russian bar near Sant'Antonio square) or attending the monthly Jotassassina party. The latter is a "straight-friendly" party and is often attended by some SISSA students.