I think Guillame is a very interesting royal. He has an inspiring sense of duty and has taken on so much already as hereditary GD. He's sensitive. He believes strongly in his values and his faith. He also seems like a real gentleman, like someone out of another century (and I don't mean that in a bad way).
I actually don't find it strange if he hasn't had many relationships. Not everyone has 15 significant others before getting married. If he isn't an experienced dater, I don't think it matters one bit because I'm sure he knows how to be the man in someone's life--romantic, strong, kind, etc. And when he meets that girl, it will come naturally. He will know.
If he has had girlfriends over the years, then bravo for keeping his private life private. It shows he has a tremendous amount of respect for the girl because the minute any woman is linked to him, her private life is over.
Anyway, I think Guillaume is a credit to his family and I'm sure he will surprise us out of the blue one day and announce his engagement to a great and accomplished lady.