It's kinda scary how many people automatically assume that married=monogamous. Not saying that such a conclusion is wrong - in most cases it is, sure. But it is the case that there are married people out there who are not monogamous and who aren't cheating. Just for example:

• Swingers
• Polyamorous couples
• People who are gender-monogamous

Not saying you have to agree with it, just that such things exist, and these things are not the same as cheating.