As a guy I say absolutely! If you are doing or saying something that you would not do with your significant other sitting right beside you then there is a good chance you know that it's unacceptable behavior.
Cheating doesn't just happen when there is physical contact. It begins with a mindset of giving yourself permission to pursue another option for happiness or excitement outside of your primary relationship. It's been said, "The kiss is the persuasion to lower invasion". Flirting and online chatting can be thought of as foreplay leading up to the possible physical act. Before you know it someone is saying, "This wasn't planned. One thing led to another...etc" Of course it did! Having "secret friends", lying directly, indirectly, or by omission smacks of dishonesty at the very least. No one wants to entrust their heart to a liar let alone a cheater whether the affair is online/emotional or physical.