Well...my theory (and don't flame for this, it wasn't me who put the idea out there in the first place, but IF it were true) is that maybe Vergil did fall in love with a normal human being, and she was preggers by the time DMC3 strolled around, and that Vergil was trying to gain more power to protect her, to keep history from repeating itself.
I mean, it's just me trying to make sense of it all. I don't believe Vergil was asexual or gay, I think he had as healthy interest in the female race as his brother did, he just didn't pay much attention to them because he already had someone to call his own. Just a theory, anyway, I mean nobody can make an accurate assumption seeing as we only really see Vergil for about 10-20 minutes of screentime, and we only see him wanting to fight Dante and getting what he wants.
My bets are that if Vergil and Dante were thrown in a room full of women, Vergil would be the one who has them all wrapped around his finger and tripping over their feet for him. Dante would be the one winning all the slaps (and bullets) in his face.
