My understanding was that what they meant by non-consensual sex wasn't the same as what we would mean by rape (notice the current generation don't use the word). Now if we imagine a society where violence is very uncommon, and the population is very sexually liberation non-consensual sex may not necessarily sound like a bad thing.

Assuming sex is something you think is always enjoyable, then 'non-consensual sex' would be the same as something you enjoy happening unexpectedly, analogous to a surprise party or being forced to have fun. So their definition of sex is sufficiently different that one would not object to 'sex' even against ones will, as 'sex' is by definition a good thing. [One would assume they have laws against violent assault that would cover sexual assault, but without the sexual nature being considered something special.]