Please help to check web sites watched on my computer in 'in private browsing' or in ' incognito window'?

OK my husband is watching porno on my back for long time. In the beginning I ignored it, thinking it's just a male fantasy and he will get over it soon. I was wrong. He all most became addicted to it. When confronted he promised that he won't do it anymore or if some time he wants to watch it he will always ask me to join him. I trusted him. Again I was wrong. He did not stop instead became smarter and sneaky. He is watching these web sites in private browsing and incognito window, also deleting history. He is making every excuse to to be left at home alone, has no time for me or kids. I don't know why he is doing it. I am a healthy, attractive young woman in early 30's. Why is he choosing porno girls over me? He is not interested in me sexually any more. He denies that he is watching porno, doesn't want even to talk about it. I know I can walk out. But I don't want to make a rush decision. I would like to save my marriage if I can. I want to know what he is watching, so I can help him or see a therapist. Please help me to find out a way to check his internet activities without him knowing.