Smart man^^^^

I know what it feels like to come between a lover and their best friend. My husband and I have been there before, although for completely different reasons. You don't feel right about it at all, but at the same time, deep down you are a little relieved.
These are YOUR issues to get over. It doesn't mean he loves you less, and you know it's not going to happen again (without your knowledge). I know you know all this in your head, but your heart is the one leading the show right now. It will take time, but you will be able to deal with this. Giving the two of them some space might be a good idea - it will prove to you that nothing is going to happen if they are together.

With that said.
Depending on the status of your relationship, if this continues to be an issue, i do feel that you are within your rights to ask them not to be together outside of your presence. Not everyone will agree with me on this, I'm sure but that's just the way I feel.

My sympathies are with you. This is a sticky situation.