I think that for most people my age and older, the phrase hook up means almost exactly the same range of things as get together. For example (though I can't promise all of these will sound natural to native speakers, as I've been thinking about the phrase enough that I'm no longer sure what's a natural use and what isn't):
- Yeah, we hooked up with Andy after the movie and we all went out to dinner. (Here, hooked up with means "met up with"; it has no romantic or sexual connotations at all.)
- Bert and Carl have been together for ages. I think they hooked up back in '94 or so. (Here, though it's a little ambiguous, hooked up is intended to mean "became a couple." If that doesn't work for you, try "first hooked up" instead of just "hooked up"; either way, it doesn't necessarily imply (to me) that they had sex.)
- Danielle and Edgar got drunk last night, and one thing led to another, and they ended up hooking up. (They had sex, or at least some kind of sexual interaction, with somewhat casual and impermanent connotations.)
But as far as I can tell, most people under the age of 25 or so use the phrase almost exclusively in a sexual context; and even for older people, using it without specifying a context ("Frank and Geraldine hooked up") often implies sexual interaction these days.
I mention this because (a) I think it's an interesting shift in meaning—I'm pretty sure I didn't think of the phrase as necessarily having especially sexual connotations, even without context, fifteen years ago; and (b) people my age and older have been using it a lot in non-sexual contexts lately ("I was supposed to hook up with X tonight" meaning something like "I was supposed to have dinner with X tonight," for example), and I keep being sophomorically amused at the potential double meaning.
On searching through my journal, I find that I usually use hook up in a different context entirely: "I hooked up my computer to my TV the other day." Heh.