To clarify - you think everyone should be free to engage in sex irrespective of anything? A man who is in a relationship and has kids is free to go and have sex with whomever he so pleases - and to do so just for the fun of it (let's assume his relationship with his partner is a good one)?
That's one heck of a straw argument you've concocted there. Neither acbytesla nor anyone else has argued that people should just be going around having sex with everyone they want to regardless of relationships or children or anything else.
Here's a life lesson for you. It's universally applicable, compatible with Christian morality, and you should apply it to all aspects of your life.
A relationship is an
agreement between people. A relationship is a
set of predefined parameters that cover, amongst other things, the sexual behaviour of those in the relationship. In a traditional Christian-derived marriage, these are the "vows" that each person recites to the other during the ceremony. Notably, the traditional vows include the phrase "to the exclusion of all others", but they don't have to. (The traditional vows also include a promise by the wife to "obey" her husband, but that's neither here nor there.)
When one person "cheats on" another, this is defined as breaking the agreement between them. If the agreement is to be monogamous, then sex outside the relationship is cheating. All relationships are based on the trust that a person has in the other to keep to the terms of the agreement. By breaking the agreement, you are betraying the other person's trust.
For the most part, most aspects of this agreement can go unsaid. The agreement to monogamy, for example, is so common that it is usually assumed. But if a person wants their relationship to be different in any way, then it is extremely important for the agreement to be stated explicitly. And yes, this means that if I am in a relationship with a person and our agreement states explicitly that we can have sex with other people, I am not "cheating on" them by doing so.
Neglecting the agreement between partners is what causes so many relationships to go bust. Because it's a betrayal of
trust, and not because of sex. If I'm in a relationship with one person, and I go off and have sex with someone else, as long as it is safe and consensual, nobody is harmed by the physical act. Sex is just sex. What
can be harmed is the trust that my partner has in my keeping to the terms of the agreement. If monogamy is part of that, then my act has potentially put my relationship at risk.
Here endeth the lesson. If you take this to heart, you will be a better person for it.