Regardless of what was meant by “cheating” in the original post, dating more than one person while in what’s otherwise understood to be an exclusive dating relationship is, if nothing else, dishonest, and therefore sinful. As long as no one is being misled, no sex is involved, and all parties are still able to continue to focus on God first, I don’t see a problem with going on dates with different people and avoiding an exclusive dating relationship.
Having sex outside of marriage, as Br. Rich says, is fornication, which is also a sin. Having sex with someone who is not your spouse while in marriage is adultery.
Dating someone other than your spouse while married denies the purpose of the sacrament of marriage. Marriage is meant to unify husband and wife. If you go on a “date” with someone else while married, it seems to me that you would be denying that function of your marriage, because to go on such a date you’d almost certainly have to prioritize some lesser commitment over your relationship with your spouse.