My (17M) girlfriend (18F) wants tattoos and I don’t like the idea.

Recently my girlfriend of a little over 2 years turned 18, and as a gift her sister offered to get her a tattoo. I’ve known she’s wanted tattoos for a while, not many, but I still find myself not able to accept this.

First of all I want to state that this is the girl I want to be with and I know it. Second of all I want to make clear exactly how I feel about the idea. I can’t think of a reason to hate tattoos, but the thought causes the exact feeling in my stomach when i drop on a roller-coaster. Third, I know that it is her body and her choice, i am not denying that and i don’t want to do or say anything that suggests otherwise.

I feel that it is not a good idea to get a tattoo so early in life. I also know that I won’t be attracted to it, and that, knowing myself, I will resent it. She has told me how she feels about getting them, and that she doesn’t want many. I’ve also told her how I feel, but just that i don’t like them, when In reality i hate them. I want her to do whatever she wants to make herself happy, and I know in telling her how I hate the idea she will not get them as that’s just who she is. I don’t want that to happen, but I think that if I don’t tell her it could lead to quiet resentment that might destroy what we have.

I’m grappling with many ideas of why I don’t like them, but I cant pin point the exact reason
1. My fear of change that I’ve always had since I was younger, especially one so permanent as this. This comes as well at a time when she is going to college, so it makes it even worse.
2. I don’t find them necessary to commemorate someone or something, rather just unnecessary markings that don’t mean anything.
3. I love her the way that she is, and I’m scared that i won’t love her if she changes herself so drastically on purpose. (This does not apply to things like age, scars, etc. This feels different because it’s purposeful.) This comes especially difficult as she is the first person who I’ve said I love you to.
4. The way I feel the most love is through physical touch, and a tattoo might not feel like her.

I’ve tried talking to her about this and it hasn’t worked. She’s explained the reasons behind wanting them, that it wouldn’t change a thing about her personality, etc. I’ve even tried to let her explain what she wants in detail so that I could get used to it, but I still feel the way I do and I have no clue why. I really wish I didn’t.

I don’t want to tell her how much I hate them and that i i find them unattractive as she will certainly not get any, causing resentment on her part, but if she does get them I don’t know how I will feel and I am scared to find out. I would also like to make it clear that I know it is her body and her choice. I really need advice on how to communicate my feelings to her without her sacrificing her happiness for my own. I would also appreciate advice on how to come to terms with how I feel and accept it. Thank you!

TL;DR
My girlfriend of a little over 2 years wants tattoos, and recently turned 18, so get sister offered one as a gift. I do not like them, but I have no clue why. I am scared to tell her as she may not get them, but I also don’t want to resent her for getting them. I want her to do whatever she wants with her own body, but am scared how I will feel if she goes through with it. I need advice on how to communicate this to her and how to accept that she wants them.