I wish people wouldn't say that, because it's not that simple. And I know, I took basic Latin. TWICE. Don't make me drag out my Wheelock's here!V was originally written as one sex, generally assumed to be male as the "us" ending is traditionally male for latin words and he's referred to as "man" in a couple of the early strips
-ius is a nominative ending for the second declension. The second declension *does* primarily consist of masculine words, yes - but not all words in the second declension are masculine.
To make it worse, -us is also used as a nominative ending in the third declension, many of which are feminine or neuter. Heck, even virtus (manliness) is feminine! (This is because it's an abstract noun. Vir, man, is masculine, as you'd expect.) And of course, -us is the nominative ending in the fourth declension, which *also* includes feminine nouns (such as anus, meaning hag - NOT the word you're thinking of, that's in the second declension!), but there are so few fourth declension words that it hardly matters.
And of course, not all words in the first declension (ending in -a in the nominative) are feminine. Two that spring instantly to mind are nauta and agricola, sailor and farmer.