Has it not occured to you that there's probably a reason why she got it behind his back?
Look how much he's freaking out over her getting a tattoo, and they've only been together four months. He's a whiner.
Yes, there probably is a reason: she's a selfish, spoiled girl!
If she'd sat down and explained "I understand your concerns but I've decided to go ahead with it anyway. If that means you don't want us to be together anymore that's fair enough" then that would be ok, but by going behind his back she's demonstrated that she doesn't give a toss about his feelings or opinions.
It's perfectly possible that the implications you made about the OP are true: that if she had told him he would've lost it and tried to forbid her from going. His OP was written in a calm and reasonable manner, but we dont' really know the guy - he could be an obnoxious, violent and abusive ****** in real life, who might've given his gf a black eye for Defying his Will!

But somehow I think it's more likely she went behind his back because she doesn't really give a damn about his opinion, and tbh I would've kept away from such a selfish girl.
Who's making assumptions now?
None of my friends or family have tried to stop me getting a tattoo, or given me any grief about them.
OK, I apologise, I drew the wrong conclusions about your friends and family because I happened to have noticed you're quite vociferous in threads where people say they don't like tattoos.
As I said, I don't have a problem with body mods in themselves. But when people start turning a tacky little tattoo into a mega-politicised issue about their freedom to express themselves through their bodies, and act like stroppy teenagers whining about how people "oppress" them if they say they find tattoos unattractive, then I get annoyed. There's people who get tattoos because they genuinely find them aesthetically pleasing, but there's also people who just get them because they enjoy the attention, whether positive or negative, and will do their best to rub it in your face and provoke comment so they can "take offense" if the comment is negative.
I don't think you're one of the latter crowd, because I remember you showing some of your tattoos and they were pretty tasteful, but the way you're standing up for this girl whom you don't know despite her boyfriend's expression of quite reasonable doubts about it. This isn't political, it's not your fight, and tbh I find it hard to understand why someone who gets the kind of tattoos that you get would jump to the defense of someone who wants to get something as tacky as what that girl got!
I do agree with you that he doesn't own her, and she's well within her rights to get any kind of tattoo she wants, even if it means he'll leave her. But what kind of person would rather have her football team's badge tattooed on her back than be with the person she (supposedly) loves? OK, so she doesn't care if he thinks it's unattractive. She doesn't find his fears about it being the source of comment or getting them into fights with football hooligans valid. All that is fair enough. But what hurt the OP wasn't just that she got it, but that she didn't even bother telling him, as if whether he leaves her over this or not is not even an issue for her! Why is that "whining"? I would certainly be hurt if this happened to me: she showed him not just that she disagrees with him and is going to do her own thing despite his opinion, but that she simply doesn't give a **** about his opinion or about his being with her or not at all!