I think you're right on both counts, but the lack of cowboyishness is probably the reason that American (and those who have bought into American stereotypes on the matter) historians especially see him as feminine.

But I know exactly what you mean. Saint-Just strikes me as one of the least feminine men of the 18th century. (Considering what the popular styles and mannerisms of the era, that's not saying much, but I think you get my point.)