It feels like it should be obvious but surprisingly there's different schools of thought on this.

Being in a relationship does not give you the right to share someone else's secrets with your partner.

I'm not talking about anything wildly illegal or anything like that.

It doesn't matter how small you might think it is. It may have taken a lot for that person to share it with you. Turning around and telling it to anyone else isn't cool; even if that person is the love of your life.

Think about that functionally. You are expecting someone else to maintain a level of trust/integrity that you've just proven you don't have by telling them.

I've heard the argument that there shouldn't be privacy or secrets in romantic relationships. Sure But between the two of you about yourselves. Information outside of that should be treated with great care. (honestly you are entitled to privacy even in a relationship but that's a whole other post).

Your partner is not entitled to hear about whatever anyone else has told you.