If you genuinely believe him to be essentially a good kind decent man, of which heaven knows there are precious few, I don't see why you can't just ASK him about it. It will be awkward and embarrassing, certainly: but all this talk of confrontation is needlessly aggressive IMO. Do him the justice of having a frank conversation. ANy subterfuge or sneakiness on your part would hardly elevate you above him, would it?
It's also worth remembering that he shortly to become a father. Is it for the first time? The transition of seeing one's partner change into almost a mother must be challenging. He may feel that it would in some way be 'wrong' to impose on you any fantasties that you might be having - he might not, at the moment, want to associate you with anything he feels is a bit 'dirty'.
I am constantly at a loss to understand why the MN 'rule' tends to be to confront, to spy, to try and wrong-foot or trap. An honest conversation, possibly assisted by a glass of wine or two, is simply the only way forward.
Human sexuality is very complex & hidden, and Lord knows there are a thousand sexual proclivities more strange and more deceitful, that would not threaten a relationship. THis is not an insurmountable problem. It's odd, I grant you - which is why you need to counter it with something as straightforward and open as you can. Best of luck!