This is the one I remember (he may have mentioned similar things in others, not sure). It's from a German magazine so what he said was translated into German and then translated back into English by a GDCer, so it's obviously not his exact words, but you can get the idea:
Billie, what does your wife say about songs like "Nightlife"? I mean, you describe in that songs your fantasie about getting seduced from a stranger.
She thinks it cool. She says, I'm sounding young and fresh..just like how I am. And "Nightlife" doesn't only describe the thing with girls, it's about stuff like drugs, alcohol and bad influence.
Is it easy to seduce you?
In this song, I just show my fantasies. My home, my family is a safe place for me. I would never risk to lose it. I live in a beatiful realationship with my wife and that since 18 years. But as a musican, you need to think different. There are things I wanna drink, thinks I wanna take and people I wanna look at. And my wife bans stuff like that. Beeing a healthy person, a good husband, a good father and to have fun in your job. That's the challenge I accepted.
(thread with full interview)
And yeah, I think it's always been pretty obvious that Billie writes songs inspired by previous girlfriends as well as his wife (especially ones about "the one who got away", being left by someone, or an exciting but volatile romance), and he's always been totally open about it as well. Don't see what the fuss is about really, it's just writing songs inspired by stuff he's experienced in his life. And all his proper romantic songs are about his wife anyway!