My spouse has been involved with someone online. It feels like an affair, but they say I'm taking things too seriously. Am I overreacting?
I doubt it. Any word, feeling, touch, or desire that belongs to you but is given to someone else falls into the category of infidelity (whether or not it becomes an affair) and will certainly feel like a betrayal to you.
Today's technology allows for easy access to online relationships. These relationships can vary in many ways: some involve only writing through email, chat, or message boards; some include pictures or video; some involve real-time video and talking. The level of contact varies from text-only writing to arranging face-to-face meetings, but all levels of contact can result in intimate communication and interaction that was promised to you alone.
There are websites dedicated to helping married people find relationships outside their marriage. Instead of helping couples overcome their problems, they provide an easy escape. In my experience, online affairs can be nearly as devastating as offline affairs, sometimes more so. You should approach them like other kinds of infidelity and get help working through them.