Spouse treason? Lol. First time I read it like that.

The best way is treating them well and looking out for them, but also being attentive to their activity. Try to trust them, try to be trusted, don't behave jealous or you'll cause a reverse-psychology reaction in which they'll cheat on you because of that.

If your bedroom is dead (if you don't have sex), both of you will have a strong tendency to discharge your sexual tension in some other place.

The lesson is, as long as you're a good couple and you're attentive and careful, there won't be much cheating, but if you are insufficient in a few aspects, trust that your partner will look to take care of those aspects with some other person.