Did he say, "Hun how do you feel about trying something different to maybe see if it is something we could maybe have fun with"? No matter if it is about watchig porn films, phone sex even, regardless what it may be. Each person is different & in long term relationships, boredom can happen with the best & most loyal people. That is not wrong to have happen & or for one to express to the one he/she is sharing life with.

The point is they believed enough in themself, their relationship, & their partner to even discuss the matter. I think should be looked at as that. Regardless of likes or dislikes at the very least it lets each know what the other is feeling & typically from there something can be resolved. If the partner just pushes away & has nothing but negitive feelings for that to even be brought, typically the one who is seeking something different, if they cannot even discuss the possibility with the one they share live with & trust, they feel rejected, feel it is or may be a one sided relationship, & are left to do for themself, for some to cheat to se if is what they really seek. That is sad, because it could have been prevented.

But no question, if one does any of what you mention alone without their partner or partners knowledge and ok, that is cheating, no matter how one spells it or defends it, it is cheating.

This leaves you to decide where to go from here.