Is it wrong for a married man to correspond via email and phone with another woman behind his wife's back?

I helped my husband set up a page on classmates because his 25th reunion was coming up. We checked for messages each night because it was attached to his home email. About a month later, my daughter was at his desk at work and an email from a woman about 30 miles away came in apologizing for not returning all his calls and saying they should meet for a beer (he graduated several states from where we now live, which means he looked for someone who was somewhat attractive and in the area). I discovered that he had called this woman several times on his work cell and emailed from his work account. When I confronted him, he stated it was all "innocent" but it was all done behind my back and I am still angry. Why choose someone in the area? Why do everything covertly? Why choose someone attractive?