Not very big, but I'll show it to anyone who asks...
I've done this with several ex partners and it really turns me on. I have a couple of rules though that help me avoid feelings of jealousy or insecurity in the relationship.
1) The other male must be a complete stranger to both of us.
2) We can only meet each guy once and never meet again.
3) I don't mind what my lady and a stranger do together while I watch, but don't want to see them kiss each other passionately on the mouth. That's something I actually find more personal and intimate than just sucking and fucking together.
This may seem weird to others but it works for me and has led to a lot of fun encounters.
I would and I have with a recent ex. It was our dirty little secret, but now I've posted my confession.