I could care less what others do and if they are fine with their partners watching it, each to their own blah blah. But I feel a bit sorry for the OP, many of the posts are almost accusatory. She has clearly said she isn't ok with this and checked his phone bc of the message. I don't check DH's phone, or computer or anything at all for that matter. But in the context of her story, that message probably would have raised my eyebrows tbh.
He is watching it behind her back. He is neglecting her physically and emotionally and his behaviour may well be suspect. Not saying he is cheating but I understand why OP may be worried about it. Telling her she is at fault here is unfair imo