Feeling unwanted:(

My fiance just admitted to jack in off in the shower ! I don't understand why he would do that when im always willing and the one most of the time asking to have sex why do men do that or don't they ?Is it normal for a man who has a women more then willing to have sex jack off instead? I know ive gained a little weight but I did have his baby ! I feel heart broken a disgusting :( am I blowing it out of proportion or do a lot of men do that? Does any of you women have this problem and how does it make you feel and or how did you deal with it ? And for the men do you do this if so why ?and are you thinking of another women ? Please help I feel so sad and I don't even want to look at him because im so hurt :( thanks for the advice .