To the best of my understanding (as a relatively new Korean learner) nouns without plurality particles can be taken as singular, plural, or as the general concept of that noun depending on context. So, 남자 can actually refer to a man, a group of men, or to the general concept of a man or men. For instance, "남자 돼지야" could mean "Men are pigs" in general or "The man is a pig". 들 is used when you want to make a noun explicitly plural, likely to remove some sort of ambiguity or apply more context.
또=tto pronounce the tt as if you are tutting and o very posh 는=neun pronounce the eu as oo