Pretty sure Duncan won't date a woman again - but I don't think he's a hypocrit for saying he's bi, I just think it's the natural thing to say when you've been with plenty of women in the past (including conceiving a child with one), but now want to date guys. If he outright said he was gay, that would 1) imply that he was living a lie beforehand, and 2) be a pretty rotten thing for his child to live with. Saying that you're gay after fathering a child just screams 'mistake', which isn't nice for a child to hear.
He's getting very into the gay 'lifestyle', which is why I don't think he'll go back to dating women. But that doesn't mean he's not attracted to them or didn't enjoy being with the ones he was with. If you've experienced both sides of something, you're always going to prefer one side and want to pursue that. Personally I think it would make life more difficult if you kept switching between preferences. If I was with someone like that, I'd always be worried that their 'phase' would end and that they'd dump me for someone of the opposite gender.
As for people coming out as bi and going back into the straight world - Lindsay Lohan jumps to mind. I mean, I know her life has more or less been a complete train wreck anyway, but she was with Samantha Ronson for a while, wasn't she?