Don't tell tomboys that they should be offended by the term "tomboy". They don't like it.
I get it; the term "tomboy" implies that their interests aren't normal, and you want to protect them from labels and traditional gender roles and all that. Unfortunately, if this is a tactic that makes sense to you, there's something fundamental about tomboys that you simply do not get.
I can think of no better anecdote to illustrate this concept than an encounter I had a few years back with one of the most boyish girls I have ever met. The two of us were involved with the same theater production, a project that involved most of the cast camping out at the theater between shows.
One morning, this girl showed up for breakfast in a pink dress; this caused quite a stir, as it was terribly uncharacteristic of her. It came out that this was the result of a lost bet from the previous night. With her arms tightly crossed and her eyes narrowed, she grimly tolerated many of our cast-mates telling her that she looked "cute" and, in all sincerity, telling her that there was nothing wrong with wearing a pink dress. She responded to this show of support by declaring, "I'm going to sit next to Micah, because he's not going to JUDGE me!"
When she did, I gave her a glance and remarked, "You look like a pink nightmare."
"THANK you," she said. And she meant it.
I almost hesitate to tell this story, since it is sure to taunt credulity for anyone in need of its message. After all, why would the girl reject the encouragement and support of the other cast members in favor of being the butt of a Christmas Story reference? If you're asking this question, there's a good chance you're part of the reason why tomboys like to call themselves tomboys.
The modern tomboy is growing up with a lot of mixed messages. They're told that girls can do whatever boys can do in a time when it's largely discouraged for even boys to act boyish. They're told that labels and gender roles are bad, and yet adults who so much as see them wear a baseball cap will ask their parents if they're pre-op. They're bombarded with validation and a strictly enforced lack of "judgement" when what they crave is the kind of conflict and competition that many boys thrive on.
The girl in my story resented people's approval of her pink dress because she had something that often gets derisively called "male ego". She didn't want people to find her "cute", because the people she looks up to most are never cute. She's more comfortable being seen as ridiculous in such a dress, because being the kind of person she wants to be means being vulnerable to ridicule.
Trying to avoid gender roles is a valid pursuit. The irony is that constant validation, avoiding labels, and even caring about gender roles are all distinctly feminine gender roles.
The bottom line is that, if you are offended by the word "tomboy", the word doesn't belong to you in the first place. Yes, it implies that there's something weird about you. The same can be said of the word "queer" (in fact, that's the literal definition of "queer"), and people are laboring to attach it to the end of an already cumbersome abbreviation anyway. Tomboys don't want to care about such things. Indeed, many of them love the implication that they aren't normal girls; they don't relate to the "normal" girls in their lives. There's something that they need that they aren't getting from them. And, if you insist on enforcing your worldview on them, you are part of why women always seem to talk about how they "used to be" tomboys, and not how they still are.