I would like to make a mod that changes what the game reads as cheating. I think that cheating should only count if the sim is in a relationship. Does anyone know how to go about this or what file it would be?

Thanks.

Easiest way to get started is to look at another mod that modifies the same system you want to change. Not quite sure from your question which game system that is... are you concerned about the romantic reputation system that came with Generations? If so, you can take a look at this one: http://www.modthesims.info/download.php?t=446164 That's an XML mod, which is a pretty straightforward type of mod to make.

Twallan's Woohooer is a script mod that modifies a lot of other aspects of the relationship system, so if you want to go beyond what you can do with XML you could look at that.

You can find tutorials here on both XML and script modding, if you're new to it