Yes. Although in this society the institution of marriage isn’t always honored, marriage alone isn’t a guarantee of faithfulness.
Married couples unfortunately do cheat. It’s a personal decision which can be influenced by partners but ultimately dependent on individual decisions. Faithfulness starts with each person within a relationship. So without marriage a couple should be faithful if they have made a decision to be monogamous.
It would be saying prior to marriage you will cheat and then once married you will change. I know plenty who think like that and are misguided. They find out that the person they are with actually lacks the integrity to be faithful in general. Remember if you have integrity you do the right thing even when no one is watching.
People need to know their integrity line or meter and find someone who has the same. Some people don’t see cheating as a deal breaker. Some people list certain sexual acts as a deal breaker. Thus that’s why some have viewed marriage as just a piece of paper. To that I say it’s a personal decision. If you are okay with having a faithful mate but don’t want to take the plunge I say carry on. If marriage is what you want then do so but don’t think a marriage certificate will keep your marriage from cheating.
Faithfulness is a decision to make between you and a partner married or not. Only you and that partner are in control of your personal interactions and how you honor one another.