Hypothetically, I would have much preferred if she had told me to begin with. So many more possibilities, there. Not particularly possessive of someone else's emotional attachments so long as a decent enough connection remains between me and them, but at least an invite would be nice, y'know? If I had somehow ended up in a legally binding marital contract with someone they would have known from the outset I could probably be talked into a threesome without that much trouble.
Probably better to know, at the absolute least if the side relationship is impacting whatever arrangements your main one has, though. Depending on other aspects of the mainline relationship there's even odds it doesn't really matter (not like you can't love more than one person, et al, and depending on the logistics aspect it's entirely possible to be married and give zero shits who the other is boinking or has emotional entanglements with, so long as the bills are paid and the kid happy or whatever), but you can't really be sure without awareness.
If there's no effects, though, and otherwise y'all's relationship is about as it ever was and as strong as desired, though... eh. The difference between something that isn't happening and something that is but has no effect on you is no difference. Still nice to know, but if you're both still getting everything you want out of your own relationship, it just... doesn't actually matter, yeh.
Brass tacks level, in complete honesty in the end I'd want to know if only so I could ask for pictures. It'd be better from that angle, and if absolutely nothing else about the situation pinged my concern that angle would still be left over.
E: Though, uh. Definite caveat for the identity of said other critter. If your wife is going down on a twelve year old, ferex, you probably really want to have a heads up. Joint and several can be a sumbiiiitch.