I agree with what everyone has already posted, but something that has not been mentionned is the damage a bored and/or ignored dog can do. You may end up with a really destructive, badly behaved dog, by no fault of his own. On the other had, you may just end up with a very depressed lethergic dog. Neither seems like an option you would want.
Finding my two dogs coincided exactly with moving in with my boyfriend, who at the time really didn't like dogs at all. To make things worse, the dogs had sever seperation anxiety and destroyed a bunch of his stuff, and on top of it all were really hyper, they couldn't sit still for more than a minute. Needless to say this caused huge fights between my boyfriend and myself, but within a month or so you could tell he was getting sucked in and now, three years later, I don't for a minute doubt that he would throw himself in front of a car for them.
Try getting your girlfriend to go for walks with you and your dog, involve her in your dog's world, she might start to warm up to the idea if she gets to know your dog as something other than just a hassle.