nice response Rowan
i have heard of a few spells to prevent your boy/girlfriend from cheating on you, but if this is a major concern i feel talking about it and couples counseling might be a better idea. a spell of this nature to stop him/her from cheating [while noble] is still messing with their free will, and it could backfire and/or wear off eventually. if you go the 'love binding' route, this won't prevent your lover from cheating, they're just going to be drawn to you forever. so if you cast a binding, and you break up a year later, don't be surprised if your now ex keeps appearing in your life years later. [meaning you're sitting at a bar in another town, and he/she happens to walk in, instead you two constantly getting together and breaking up]
if you're asking for a spell so that he/she only desires and want you, and never looks at porn, celebrities, or a random man/woman at the mall, i don't think there is as this is a part of human [and animal] nature. you need to accept the fact this happens, it doesn't mean they'll leave you.
whichever i recommend talking it out before going behind your lovers back and casting a spell on them.