She got the tattoo removed, right?
Why she got the tattoo? Young and stupid. If I wasn't riddled with OCD about ink I would have all kind of ink I regret. I grew up in the body community and stick to a think about it for six months to a year before getting it done. If you still like it, do it. I bet she gave about as much time thinking about getting that tattoo as it took her to get to the parlor.
Why she won't talk about it with you? How did you react to the news of her experience with a micro-penis? If you perved out or weirded out about it she might not be comfortable discussing the issue with you. AND this was over 20 years ago. She's over it. The information is new to you but old hat, and dealt with, for her. Why do you care so much about a guy she had sex with like two decades plus ago? One she's clearly left behind with very little thought and has been with you for the majority of her adult life?
Also.. being forthcoming with all the details in your life is not a requirement in a relationship. Just because you would answer any question, which you can't honestly say until you have been asked everything, doesn't mean she owes you tit for tat. You've found a topic/time in her life she is not comfortable talking about with you. Be a grown-up about it and let it go.