I'm a marriage and family counselor.

What I tell my clients is that cheating is subjective.

Everyone has different opinions as what counts as cheating, with many people drawling a line at physical intimacy, such as kissing, or obviously sex.

Cheating is primarily about crossing or betraying boundaries.

If you feel uncomfortable with what she's doing, you need to tell her that, and she needs to respect that.

Her choice is to either respect how you feel, or to leave. She is free to disagree with how you feel, but she is not free to stay with you and force you through that.

If she continues to go behind your back and just not tell you, then it's cheating. No matter what is actually happening.

If she doesn't respect your boundaries, and does it behind your back, then you are free to leave as well.

What's important and needs to happen first is communication.

My personal opinion though is it does not look good. Trust your instincts my guy. You deserve better than this.