When we finish Mission 19, there’s a cutscene where there’s a little conversation between Dante and Vergil. In that little conversation,

Vergil asks Dante “Nero… is my son?”,

Dante replies “Yeah, dumbass.”,

and quickly asks Vergil “Remember through that thick skull of yours?”,

Vergil replies “Well well… that was a long time ago.”,

then Dante laughingly says to Vergil “I guess you were young once too.”.

When Dante says to Vergil “I guess you were young once too.”, he means that he hints that Vergil wasn’t the only one who was a young once, Dante was a young once too. In other words, Vergil wasn’t the only one who had sex when he was young, Dante had sex too when he was young.