How do you not know when your mate is screwing around on you?

I can answer that one.

Exhaustion is one reason. When my ex cheated on me, I was getting 3-5 hours' sleep a night. He was unemployed and (he told me) physically unable to do the housework because of chronic back pain. So he had lots of time on his hands, I had less than none with both the breadwinning and the cleaning to do, and he was already acting grumpy and weird all the time anyhow because he was bored and in pain. On 3-5 hours' sleep a night for several months on end, you're lucky to notice if the furniture got rearranged while you were out, never mind any of the traditional "cheater" symptoms.

If the spouse is already acting weird because there is already an external stressor (no money, lost a job, something else), it might not click. One spouse can be making efforts to hold everything together, while the other decides they can do WTF they want because they're already unhappy. The decent thing to do is to break things off and then find a new partner, but a lot of people simply don't do the decent thing. They've already left the relationship in their heads, but they never bothered telling their spouse, and it's easier to keep the peace if they don't bother being honest.