Also, if you -know- it would upset him, you've answered the question yourself.
This sums it up. And it points out that the real question has nothing to do with cheating. The real question is: When my partner finds out what I have done, will he be hurt/upset/angry?
Clearly, the answer in this case is Yes.
Doesn't matter if what you did was wanking, mutual bj, flogging each other with banana peels, establishing a secret second relationship, painting each other with peanut butter and licking it off, spending your joint savings on medieval torture devices, trying the "cocaine and cigarettes" diet from the 80's, moving suddenly to Peru (unless you are already in Peru), or buying GM stock shares.
The point is: Don't do things that will hurt your partner!
The other point, already made several times in this thread, is: You have every right to negotiate outside sexual activity (or anything else, see above). You and your partner can mutually decide on the rules or boundaries.
If your partner won't go along or refuses to negotiate the terms, then you will have to decide whether you can live without it. If you choose to stay with him and NOT do these things, try not to resent him for his feelings, because that will surely poison the relationship. Don't agree to terms you cannot live with contentedly.
