Yes, I think it's part of the Clan warrior thing.
In the IS, getting sex is a hard thing, and can require a lot of time to acquire.
The Clans do away with courting, dating and all that other mushy stuff, to make time for the important things like training and warrior-ing.
Sex isn't trivialised, it is still an intimate thing, Clanners are human after all; it's just that it's seen more as a stress-reliever, something you can do with your friends (or family, but the Clans do tend to downplay the family thing a lot in their doctrine).
It keeps morale up; there's no sexual frustration, there's no penis envy, there's no sexual rivalry; you just get off with an old (or new) friend and go about your day with a big smile.
Also, it means unisex showers and toilets are the norm, with both sexes using them at the same time, which makes building such facilities that much easier.
The civilian castes are fairly promiscuous too, though they get saddled with arranged marriages (the low-tech version of the eugenics program). As long as there aren't any accidental pregnancies though, people don't really give a rat's ass who you play around with. As long as you keep pumping out the sanctioned, genetically-superior children, it's all hunky-dory.