So what's the moral of that story? I like morals, I think stories should have morals.
Moral #1: Infidelity is in the eye of the beholder
I remember an old episode of Rosanne where the boyfriend of one of the daughters is all upset because she's mad at him and he doesn't think he did anything wrong and, in fact, it's clear that he didn't, the girl is irrationally upset. Dan is trying to explain the reality to him:
Boyfriend: "I don't understand why she's angry ..."
Dan: "It doesn't matter."
Boyfriend: "I didn't do anything wrong..."
Dan: "It doesn't matter."
Boyfriend: "She's being unreasonable ..."
Dan: "It doesn't matter."
Boyfriend: "I'm right..."
Dan: "It doesn't matter."
This is reality. You may not think it's fair -- it doesn't matter. You may think you're right -- it doesn't matter. Everyone you ask may tell you your actions are justified -- it doesn't matter.
What matters is how your partner feels about it and how they'll react. If it's something you feel you have to hide from your partner because they "won't understand" and they might be mad, guess what? You probably shouldn't be doing it.
You and your partner probably haven't sat down and laid out every specific act that you'd each consider infidelity and a betrayal. That doesn't mean that there's not a list in each of your heads of what constitutes that. If you do something on their list, it doesn't matter what you think or what everyone in your circle of friends thinks -- your partner is going to react as though you were unfaithful, because that's how they see it.
Do you think oral sex is being unfaithful? Probably you do (if not, insert some other act). How would you feel if your partner got a blowjob from a hooker? Would it make a damn bit of difference to you if they then said: "But I didn't think that was really sex ... it's just a meaningless blowjob"?
If you're in a relationship where you supposedly love and trust each other, the hesitation you feel about sharing your actions with your partner are probably a good indication that you're violating their trust.
Moral #2: Infidelity hurts
You love and trust your partner ... or, at least, you did once -- presumably that's why you got married or entered into whatever committed relationship you have.
If they feel (see Moral #1) you've been unfaithful and betrayed that trust, it will hurt them. How can you love someone and be willing to hurt them?
(Silly question in a BDSM forum, but there's an implied "hurt them in a bad way".)
Even if you don't love them any more, doesn't the love you once felt for them demand a certain amount of care for their well-being?
Moral #3: Infidelity has impacts beyond the two of you
The feeling of betrayal will color your partner's relationships for years. You're hurting not only someone you love (or loved), but innocent, unknown people who'll pay the price for your actions when they have to deal with your partner's new found trust issues.
Even if you now hate your partner, you'll be hurting people you haven't even met.