Justifying tattoos: I use my money for things that matter to me. Functionally speaking, this means that a significant chunk goes into savings to buy future freedom of decision, a decent-sized chunk goes to pay the mortgage on a house I love to live in in the place I want to live in, some of it goes to maintain a car, some of it goes to daycare and excellent food, and some of it goes to things I care about. I evaluate my spending decisions based on whether I think the thing I'm spending money on will make me happier long-term and me more long-term satisfaction than having the money in savings.
In practical terms, this means I'm willing to pay a mortgage on a relatively nice house that isn't biking distance to anywhere, have kids, spend money on books, not eat out at all, drive an older car... generally balance my spending in a way that suits my values, my life, and my long-term satisfaction (as well as my husband's. Obviously.)
So: my tattoo. I have it. I like it. I have plans for another one, probably in a few years (due to both finances - the artist I want is excellent but has a waiting list and is $$$ - and due to pregnancy/breastfeeding planning). I think it's worth the money for my happiness and satisfaction. You can feel absolutely free to disagree with your own body and your own money. *shrugs*
Anyone else is free to disagree... in the same way that you're free to think that I should do something different with my hair, wear different clothing, spend more/less on my house, manage my relationship with my husband differently, parent my toddler differently, have more/less kids, whatever. Judgy judgers will judge, basically. You can think it, but I don't have to listen to it. ;)