Love To Watch Again

Valentines day last year I treated my husband, and myself, to a MMF threesome.
Only most of the sex happened between my husband and our guest.
I couldn't believe it at first as they sucked each others cocks, then my husband's took our guests cock deep inside his arsehole.
However watching my husband being fucked turned me on immensely, so I had him tongue my pussy as he was being taken from behind.
The evening turned out to be a great experience for us both as eventually both men double fucked me. But I couldn't stop thinking of how beautiful and horny my husband looked receiving cock up his rear.
Our sex life has even improved over this past year with my husband being far more adventurous, like the other day when he pulled over down a country lane at night and fucked me over the bonnet of the car.
The thing is, recently I found out he and the guy we both enjoyed together have been fucking on a regular basis.
It hasn't impacted on our marriage in any way, but I'm a bit miffed he hasn't seen fit to let me know.
I'm guessing he's being fucked every week and I'd love to experience them having sex again, but how do I brooch the subject without letting him know how I found out.