If you think you have to spy on your partner, why, exactly, is that person still your partner?
Seriously ... If you spy on someone, and they prove guiltless, YOU are guilty. When they find out you spied on them (and they will, Murphy says so), YOU become the bad person, and they dump you.
If YOU prove right, and they are guilty, you're gonna bail anyway.
So once someone deploys this kind of thing, a break-up is probable ... Surely it'd be better for all concerned to just split up and move along with life, rather than drag out the inevitable?