So this is a study of people who are in their forties who are married or cohabitating, but it doesn’t look like the length of the relationship was also a factor — is that right? ‘Lasting couples’ implies that this data was gathered from 40-somethings who have also been married more than, say, 15 years (versus those…

Yes.

I would always rather know. I wouldn’t necessarily end a relationship over being cheated on if my partner was honest with me about it (depending on a million other factors, obviously), but I would absolutely end a relationship if I was cheated on and found out by some means other than my partner coming clean.