Use a marriage therapist's 3 S's to determine if your platonic friendship has become a full-fledged emotional affair

Emotional infidelity is murky territory - which is why it's easy to lie to yourself about the way your relationship with a "friend" is progressing.

Yes, your heart skips a beat when you see a message from them and yes, the time you spend with your partner is starting to look increasingly dull. But it's not like you've confessed your love to each other or anything, so you're not guilty of any wrongdoing. Right? Right?!

This is where it helps to bring in someone more objective. Sheri Meyers is a licensed marriage and family therapist and the author of "Chatting or Cheating" - she's heard it all when it comes to infidelity.

Meyers told Business Insider that, if you're really not sure whether you're currently involved in an emotional affair (or "emotional sex," as she calls it), you can use the three S's to figure it out: secrecy, shared intimacy, and sexual energy.

Let's break down those signs.

Secrecy means "you're starting to become secretive about how much time you're thinking about, spending with, interacting with this friend," Meyers said. "You're starting to say things to the person that you aren't telling your partner, and you wouldn't be interacting with this person when your partner's around."

Here's a classic example of a secretive relationship: You're texting your friend when your partner walks into the room and you immediately put the phone down.

Shared intimacy happens when your friend becomes your "emotional confidante," Meyers said. In other words, you've started telling your friend the type of things that, at one time, you would only tell your partner.

Sexual energy is another key component of an emotional affair - even if it doesn't manifest in anything physical. "You are drawn to this person, whether you act on it or not. There's this underlying sexual energy and chemistry," Meyers said. "It almost feels bigger than you."

In fact, Meyers said, sexual energy can (and often does) exist even if you've never met the person, and only chatted with them online.

Ultimately, the three S's indicate that your friendship is evolving in a way that could threaten your commitment to your partner.

As Meyers writes in "Chatting or Cheating," "Most people feel the three S's should be reserved exclusively for their primary relationship, and often feel betrayed if they are shared with anyone else."