IIN he wants me to stick my finger up his butt?

I do this to my wife all the time at first she didn't want it at all but I got her to try it and every time we have sex it always happens I stick my fingures up her butt and oh does she love it, it always feels great it's amazing I always do the same to myself it always feels good I would recommend trying it sounds sick but once you've tried it you'll know what I mean