I am NOT in ANY way condoning or excusing an extramarital affair. I heard your question to be "Why did this happen?" Or "How could he?" So I'll offer my thoughts.
I believe the answers you are looking for may be found in Gary Chapman's "The Five Love Languages". This book was recommended to me a short time ago and I actually had a copy at home (well, my wife did) that I had never thought of reading. Everyone, male and female, has their own primary love language. If you or your husband aren't speaking in one another's language, Satan WILL send someone who is. My primary language is physical touch. I am assuming your husband's is too. This doesn't always mean sex, but it does include it. Although God's grace is sufficient to fill all of my needs, the temptations are STRONGER and MORE FREQUENT when I feel 'low on love fuel' or my self-esteem is low (which can only be cured by a personal relationship with God). This could very well be the same for this other woman. I don't think it is a 'male thing' although it may be more frequent in men.
What may be confusing to you is that physical touch isn't what makes you feel loved. Your language is one of the other four....Quality Time, Word of Affirmation, Acts of Service (my wife's), or Receiving Gifts. One of these things makes you feel loved. I think nearly everyone at some point in their life has encountered a person who spoke our love language better than our spouse was doing at the time. Those who are strong in the Lord are able to identify this as an attack from below and resist. But these moments often lead to 'emotional affairs' which can be just as devastating to a marriage. In either case we are allowing ourselves to feel loved by someone other than our spouse.
So was it just sex with her? No, it was your husband not 'feeling' loved at a weak moment in his life and Satan attacking. Is it just sex with you? No, it is your husband wanting to feel loved by you and he believes that it makes you feel loved too. I encourage you both (and everyone) to learn to speak your spouse's love language fluently. I am firmly convinced that God wants me to show my wife love in a way that SHE understands even if I don't.
My prayers for both of you,
J