Cheating in its simplest form involves supporting something, and then letting it down. If you see yourself as a collection of attributes of all kind, be it physical, mental, spiritual or otherwise, then your friends support most of them. It is a network of selection in which more people support, or select, the same things together (what they like, what they do, etc.). If one of your friends suddenly selects something which goes against everything else, say a radical view on things, then you are likely to completely abandon them and stop calling yourself their friend. Even more, you might make them your enemy.
Imagine the same goes for relationships, but more extreme, and with the involvement of a more relevant selection tool: sex. Evolutionarily speaking, it is easy to figure out why sex has become such an important selection tool, by the use of reproduction of genes. But through the ages, as humans started artificially modifying their environment, the separation of sex and reproduction, on a behavioural level, is apparent. Humans do not have a ticker in the back of their heads "telling" them to have sex so they have children. Quite the opposite is true, where those who enjoyed sex more in the distant past inevitably passed on their genes by reproduction, therefore producing modern humans, most of whom have a strong sex drive, and big potential for physical pleasure.
This urge is a by-product of that selection, and drives people today. The ability to control actual reproduction leaves sex in its own right as a selection tool, among others such as laughter. That is why we perceive a strong selective bond between those who have sex with each other. For example, if two people have sex, then they are perceived to support all their attributes, their looks, beliefs, character, etc. Hence, it is ridiculous to suggest people with contrasting attributes would have sex; for example, good looking people with unattractive people, smart people with dumb people, etc. Of course, the reality of it is different, because different people value different attributes more than others. Some are prepared to overlook everything for wealth only.
It is no surprise, therefore, that your partner having sex, or getting along with (both of which are selection tools), a potential partner whose attributes are different to yours, is so outraging. We see ourselves as a unique mix of attributes, so being cheated on is never going to be acceptable. Still, some people overlook it, or do it themselves without showing any resentment. Some people "get over it". The way this happens is either by diluting the act of cheating, and considering it a weaker selection tool (for example, having sex doesn't mean anything more than a physical act), or by reaching the conclusion that the partner still supports you more than whoever they cheated with.
The more sexual partners someone has, the more diluted sex becomes for them. Someone who has only had one partner all their life is more likely to consider sex very important in showing selection, or support for someone, while someone who has had sex with countless meaningless people is more likely to deem it as "overrated" and not associated with supporting anyone.
Check back next time for a post on High-end products of Human artificial selection. You know, like designer shoes and iPads.