XH2 had slept with pretty much all his female friends from home, and a lot of them were in relationships with his male friends. This freaked me out a bit when I first realised - no group of my friends has been that, erm, intermingled. However, it's clearly just how things were in their neck of the woods. A lot of things were wrong with our marriage, but that wasn't one of them. I still think it's weird, though!
So, as you haven't posted any background yet, I'm wondering if you feel much the same way OP? That nothing's particularly wrong about it, but it's just a bit out of your comfort zone?
If the pair of them are constantly flirting with each other, disappearing into quiet rooms together or excluding you with sickly in-jokes, you've got a problem. If everyone involved is perfectly at ease, it's probably a blip from the past that they've all moved well past.
Have you told him you feel odd about it?