Regarding the Ilia character and Deltans in general, I'd like to propose an alternate explanation: Gene Roddenberry was a horndog. In the 1960's, he was restricted a bit by TV censors. In the late 1970's, he felt he would have much more freedom. So he created a species who's defining characteristic was that they have sex all the time. No doubt with the intention of depicting a wide variety of sexual encounters involving scantily clad women, many of whom would likely visit his personal casting couch to get the part. A harsh assessment? Yes. But backed up by what we know of Roddenberry's treatment of women in general.
In-universe, I'm not entirely sure how Deltan society manages to function, though. One description I read of them -- from the novelization perhaps -- says that to Deltans, sex was part of literally every interaction between people and as common as a handshake. Ok, how many people do you casually interact with in a day? How many people do you shake hands with? Imagine if literally every single one of those encounters involved you having sex with them. How would anything ever get done?