Whatever works for you, I guess, but I find this incredibly short-sighted and more than a little bit offensive.
While it's not for me, I can see the logic behind it.
Some groups use D&D Night kind of like a poker night or a guy's night or something along those lines. Getting girls involved in "male bonding time" is usually kind of an invasion-of-space consideration. D&D just happens to be concurrent with this "male bonding time." It's even easier if the group started out in high school or college and has survived the ladyfriends coming and going.
I probably prefer actual beer and strippers over imaginary ale and whore for my male bonding time, but to each their own, yeah?
And I'd never pass up the chance to game with some of the coolest girlfriends (and, maybe surprisingly, coolest boyfriends....more of those, actually) on the planet, myself.
If D&D Night is your "Night with the boys," anyone's girlfriend is going to stick a big ol' buzzkill on the night, just as a wife invited to poker night or a fiancee invited to the March Madness party with Paul's new bigscreen would be an unwelcome participant.
And just as I'd be unwelcome in a girls' D&D night, or a ladies' night of beer and strippers, or whatever it is that girls do when there are no guys around (the Internet tells me it's sleep overs and sexy pillow fights, but I have my suspicions about the Internet...).