Why do mm cheat

I have been married 12 years happily I thought. I recently found out my husband has had a girlfriend for a year and a half and she just gave birth to a child. My husband and I have been trying to conceive for about 5 years and I have had several miscarriages. It sound cliche but I really was shocked because I'm like no not MY husband he would never do that. Not like he would never gets blow job in a club or have a one night stand but a whole relationship and a baby! He had no regard for my safety go in between the two of us and such blatant disrespect. According to him he does not love her and would have no prob never talking to her if it wasn't for the baby. Also he swears he is so in love with me and I am the best wife he could ever ask for so he is saying he had no problems with me. So obviously that is a lie but my question is from men who have done this and been relatively happy in their marriage why did you do it. Did it surprise you how hurt your partner would be, my husband for sure does not get the magnitude and totally does not understand my feelings. For him it might mean less that it does for me but I just can't understand the thinking or lack there of behind it. I cannot afford the mortgage on my own and the day after I found out I also found out I was pregnant. No I'm going to be a single parent alone and broke its like did he ever consider me at all. Is it possible to love someone AND do his to them?