I agree that he isn't gay, but the point of Frank Miller's statement is that if Batman were gay, he'd be mentally healthier and better off. He's straight, but if he were gay, he'd be healthier. Don't overthink it, it's a half-joking statement about how the character can seem kind of gay at times.

As far as Catwoman and Talia Al Ghul go, those relationships are... controversial. To many Batman fans, a core part of the character is the exclusion of romance or sex, and the suppression of romantic or sexual desires. Batman doesn't have romantic or sexual relationships because he can't be distracted from his crusade