It is absolutely wrong. I was with a man who I absolutely loved with all of my heart for 5 years, and engaged within those last 2. I found out about a month before our original wedding date that he didn't physically, but emotionally cheated on me with another woman online. I found out after seeing a strange message on his phone when handing it to him. My gut feeling was right, and even messaged the number where I found out more extensive information of conversations they've had. He didn't deny it when I confronted him. If I never have found out we could have started off a marriage wrong, especially on his part. He is addicted to video games (every free time he had), and it was starting to feel like he was treating me more like his maid/mother instead of his partner.
Some things I'd recommend:
Talk to her and let her know how your feeling and why. Communication is something a lot of people are gradually starting to forget about with all this technology in our faces, and it's huge in any relationship.
If it's some excitement your looking for why not make an effort and spice things up a bit. Do some fun weekend getaways. Both of you can try something new together. And lastly, see if you think maybe try couples counseling if communication is lacking on both ends. A friend of mine said that it was very helpful in trying their tips and tricks that many don't ever think about. Talk to family as well if they are a huge part of your life, so they know how your feeling.

But please, don't look for an affair. After at least trying all or any of those than you'll know whether separating/divorce (in a calm civil manner) as the last resort.