If you developed a crush on someone else while in a monogamous LTR, how would you handle it?

If your SO developed a crush on someone and came to you about it, what would your response be? If you prefer they not tell you, how would you ideally want them to handle it?

Curious to see how red and blue differ here.

Edit: I should add that I think that crushes are not that uncommon in LTRs, even really good LTRs. So "just leave" is not a great reply in my estimation.