Just because there is nothing I can do about it doesn't mean I like it.
You like it because often times, if people told you the truth, it would be hurtful. As children, we are taught not to say things like, "That's a funny looking mole on your face" to people because, even though it's true it may be hurtful. So is it a lie by omission if we don't tell our friends that we think they look too fat, or have a disfiguring mark on their face?
Lying by omission
Also known as a continuing misrepresentation, a lie by omission occurs when an important fact is left out in order to foster a misconception. Lying by omission includes failures to correct pre-existing misconceptions. When the seller of a car declares it has been serviced regularly but does not tell that a fault was reported at the last service, the seller lies by omission. It can be compared to dissimulation.
In order for this to be true, "I had sex with someone else" would need to qualify as an important fact. If that someone else didn't give you a disease, isn't related to either of you, and will never be a part of your life again, then I fail to see how it would bear any importance.
1. Giving your spouse the truth even if it may hurt them or may cause them to pull away from you is an act of love and respect.
2. Doing something that you know would hurt your spouse if they knew for your own personal pleasure is an act of selfishness, calousness, and disrespect. Having sex behind their back when in a monogamous relationship is doing just that.
1. Again, your definitions seem preposterous. How is telling someone something they don't need to know, and will hurt them, and may change their lives forever, an act of love? Your job as a spouse is to love the person you are with and make them happy for the rest of their lives. Telling them you fucked someone else, (whether it happened or not) is the direct opposite of that. The problem is not the sex. The problem is the confession.
2. I grant you that sex with someone other than your partner is selfish. I'm not sure what callousness has to do with anything. Having sex is an act of pleasure, not suffering. As for respect, that's an interesting one. People infer respect or the lack of it, based on our behavior toward them. If your behavior towards your spouse does not change, they will never feel disrespected. If you hold up a middle finger behind their back, you may think you are disrespecting them, but they will not. So who's perception is more important? That of the one doing something disrespectful or that of the person who may potentially be impacted by it? I would argue the perception of the recipient trumps the perception of the giver or denier of respect. Certainly in political situations, sometimes diplomats feel disrespected even though none was meant by the denier of respect.