My great nan and my boyfriends nan are sisters. They both had children, who were therefore cousins (my grandad and my boyfriends dad), my grandad then had my mom, whilst my boyfriends dad had my boyfriend, and then my mom went on to have me, we only just found out, is it wrong?

I kind of think so yes, but you two can't really be blamed if you never knew. It will really weird myself up I know that, and I would look at her/him in a totally different way

You're 3rd grade cousins.

Its not exactly wrong mind you.

You probably wouldn't have found out and the blood relation you have is already quite slim.