yeah, it’s not limited to sex. it’s a general term referring to putting two parts together.
these parts can be two human people ("me and my client need to hook up this week for a meeting), as mentioned “introduced to” (i got hooked up with this great new church/he hooked me up to this new concept of web-surfing), or can mean parts (they hooked up ((their sexual organs//i got hooked up with a black suv ((car crash)).
i believe in the op example, it’s referring to romantic entanglement. it doesn’t per se have to refer to casual sex, as it is often said “they were hooking up for a while” as reference to two people dating (maybe it is presumed they were having sex, but that isn’t necessarily the implication of the term).
I have heard the term “hooked up” to mean have sex with but this usage was not in play when I was a teenager (I am 55). I first heard this usage around 1990, although I am not exactly on the cutting edge of pop culture so it probably started earlier than that. All the old usages are still valid, mostly to meet (in person), to communicate with, or to establish some sort of connection to, as described in posts above.
If it’s a high schooler or college-age kid, I’d say it means casual sex.
If it’s an older person it could mean casual sex, meeting someone somewhere, or starting to date someone. Really you’d have to be in on the whole conversation to know for sure whether it means sex, but generally I think it means sex.
In college in the mid-90s, it meant casual sex (ETA: or at least some romantic entanglement. Being college kids, we assumed sexual activities of some sort, not just a little kiss, although making out with someone could be considered “hooking up,” too.) It can be used in other context, though. Like “Let’s hook up after the game at Jimmy’s” just means “let’s meet.” “Bob and Sally hooked up last night” usually implies a sexual liason of some sort.
[creaky old person voice] Back in my day [/creaky old person voice] (late 80s, early 90s) amongst my peer group it meant casual sex. “Jen hooked up with Mike” meant the two of them had a sexual encounter, of the non-relationship variety. A hook up could end up as a dating relationship, but it didn’t have to, and was definitely not preceded by dating.
If someone’s said this and I just missed it, my apologies:
Especially among the current younger crowd, hook-up is often used as a noun - for example, “Jenny and I had a hook-up last night.” This does indicate a sexual, casual, non-relationship encounter. (Though sexual could be anything from cuddling or making out all the way up to intercourse, and the relationship might develop later.) I’ve never seen hook-up used as a noun with any other meaning.
Used as a verb (“Jenny and I hooked up last night”), you have to rely on context to know what’s meant.
When I was an undergraduate in the college band, about 15 years ago, when we went on our fall tour the tuba section always ran a betting pool on who would hook up over the course of the trip. It could mean sex, but it was really an “I know it when I see it” sort of situation: Anything that indicated a romantic or sexual connection and that was known to the tubists counted: Kissing, always making it a point to sit very close in the same seat on the bus, whatever. Note, though, that a continuation of an already-existing relationship didn’t count (though whether a relationship continued afterwards was wholly irrelevant).