i propose a mandatory microchip implant with programmed microprocessor to track every movement, particularly thermally triggered circuits to monitor the physical infidelity.before entering into "love contract" it should be properly installed, and maintained throughout the love period(even after two kids).this would definitely help in these modern times where options are aplenty and both male and female have the chances to wander off very easily.
on a serious note, if  you don't trust him/her anymore, there's no point in continuing with the relationship.lack of trust is an infection which only spreads, no antibiotics can kill it.