I have done it/ do it with one particular bull my wife has a thing for beyond sex. We have been great friends for a few years (and his personal hygiene is beyond impeccable).
I can tell you now, you have never, EVER seen, felt or heard a man cum so hard than when your tongue is buried deep in his ass as he cums in your wife.
That feeling as his hole loosens and tightens around your tongue while he is climaxing is such an addictive ******* for this submissive cuck and the fact you have helped give this man an orgasm/s he will not only never forget, but also can't achieve without someone willing to submit to such a submissive act.