Tattoos look bad on women... for numerous reasons, really.
Probably the biggest factor is that women have terrible taste. In other words, they would select a very shitty tattoo off of the pre-drawn Greeting Card clip-art gallery wall of either a butterfly, a pixie, Warner Bros.' Tweety or a pixie Tweety with butterfly wings.
Also, tattoos on the small of a girl's back is trite and is getting lamer and lamer as I see it more and more. And let's keep in mind, my tastes are supreme and my opinions are actually considered fact by many scholars.
If my girlfriend got a shitty tattoo, I'd give her a tattoo of my hand in grip form around her arm.
However, I like tattoos on a girl's ankle. A little one or one that wraps around the ankle. They often taint whatever part is tattooed (like breasts or ass), so watch where you get it.
... by the way, that was my alter-ego talking.
Don't coincide this with some type of relationship control or a bizarre women's liberation or something equally as petty. If he has expressed that he doesn't want you to have a tattoo, then adhere to that request -- a relationship is a compromise, afterall... stop thinking about yourself. I'm sure if he said he was planning on dating another man and/or living the life of a pirate, you'd disapprove. In the end, if you were to get a tattoo without telling him (and you shouldn't do so), he's still love you... assuming he's not a cool-ass frat boy or something to that affect, of course.
If you really want to do some bra-burning liberation, I suggest this tattoo:
He'll love you for eternity.