I would like to know the subtle difference between hook up and have sex. I'm asking because hook up seems have a subtly different meaning than have sex: in the situations I've heard this word it seems to mean sex without compromise, while have sex is more general. On the other hand, the dictionaries say hook up = have sex.
"Hook up with" is ambiguous in meaning. It can mean to meet up with friends to hang out, but it can also mean to have sex with someone. Generally, if you are using it with a singular person
I hooked up with this person I met last night at the hoopda.
or in particular if you include "totally," as in
I totally hooked up with my friend last night.
the implication is that a sexual encounter occurred unless there is other context that suggests otherwise. It also implies that the encounter was casual and perhaps almost random; you don't usually "hook up with" someone that you have a long-term relationship with.
By contrast, if you use it with a plural person
I hooked up with my friends last night at the hoopda.
the assumption is that you mean "met up with."
"Hook up with" might be used to imply or suggest that you had sex with someone, so in that sense, it is a more discreet way to say you had sex with someone.
On the other hand, "have sex with" is pretty unambiguous and means to engage in sexual relations with someone else. It is not explicit in an offensive or rude way, but you will leave no doubt that that is exactly what you mean.