OP, of course reading people say it like it is, when it is a man you love, is very confronting. We understand that, and we understand you feeling defensive now, and wanting to defend him.
However, please truly listen with an open mind and heart to what posters are saying. This is not at all normal behaviour of a man about to marry the woman he loves. Not at all. It's something I'd expect from a man who has been married 10 or 20 years (and it is still wrong), not someone in the 'honeymoon' stages of a relationship. When a man loves a woman, he will not notice any woman, let alone look at them. Let alone search them on the net. And he does this every day? That he's scrolling so fast when you enter the room, means he knows he is doing the wrong thing.
You can say he is a decent chap all you want, but 'decent chaps' don't do what he does. He is outright disrespecting you, and even admitting to it ("some girl"). Shameless about it, and not even feeling guilty. Are you honestly going to settle for that?
How can you say he has your best interests at heart? If it was occasionally maybe, but it is a fixated daily habit. You'd be fooling yourself if you honestly, genuinely think his obsession won't get the better of him and cheat. It's bad enough he disrespects you, don't disrespect yourself. Somewhere out there there's a man who will treat you with respect. R.E.S.P.E.C.T find out what it means to you (as the song goes). Respect yourself enough to deem yourself worthy of more, and walk away. Better you do it now, before you marry and he leaves you for an OW. And it will happen. Hold your head up high, believe in yourself and the sisterhood, and leave with your self respect in tact.