Lost..

My SO and I have known each other for 8 years and been together for almost 5..  We recently had a son three months ago.. 

i use to find him on dating sites and etc. it stopped whatever.. well recently found he made a cam site.. and wrote someone a note.. 

when I confronted him about it he tells me it’s my fault because I haven’t been starting sex lately and he did it to see if I’d notice or care...

I’m so lost.. how are you gonna do that to the mother of your child..