I'll be honest,
If you're not giving him sex, and all he wants is sex, you will never be able to tell. But your brother would be able to tell. And if he does only want to have sex, then he's a dick, and may go elsewhere for it.
I'm not saying for sure he is, I think you don't have enough evidence about him cheating, girls will comment on his looks if he's a good-looking guy, that's inevitable, it's not his fault. If he's playing along, and provoking it, then he should back off, that's not nice to you, but it still isn't his fault. Your friends could have the right idea, if he has a history of cheating, or being an asshole, take heed of it, but don't end it due to that, it could easily be a misunderstanding, or it could have been a one-time thing, I know I'm a great guy, I know I am, I would never hurt anyone, but I had 1 one-night stand, after a girl broke-up with me, it got out, and I was labeled by some people as "an asshole", because they thought all I wanted was sex. (the girl and I didn't even sleep together, just made out, so it shows you how these things can escalade to be out of proportion, I'm a virgin and they think all I want is sex.)
So, in the end, don't worry about it much. There'd be more obvious signs of him cheating if he was, but I would think about what everyone is saying, if you're getting it from a lot of people that he's an asshole, and all he wants is sex, then you want to be careful of him at least. But not paranoid.
And the marriage thing, depending on how long you've been dating, it's hard to say whether he means it. When a guy says something like that, he is either saying cause he thinks its what you want to hear, or because he truly means it, rarely is there a "uhm, I'm not really sure about it" type of thing like there is with girls.
First of all dreaming of him cheating on you is deffinately not a thing to worry. Its just you dreaming about things you fear like many people do. My opinion is that every good working relationship is based on truth and respect (if there is no truth in the relationship there is no point going on with it). So if you know you say the truth to him and he does to you , there is trust ... when there is trust things work just fine. I am a bit jealous too but I know I can trust my partner and vice-versa and talk about everything its fine. If he hides something from you , doesnt want to talk about things , doesnt care about you or your needs thats when you kind of know its a problem... So finally only you know how its really your relationship and if he is to be trusted or not. It matters how he acts to you with you around you not what his friends say. So trust him or dump him! And talk things out ! Solve your issues dont pass them by they dont dissapear they just accumulate.
i used to date a guy that did the same thing and he kept telling me he wouldnt but when i checked his login on the computer i found picture of other girls so my ex was cheating on me
A lot of the time on lookers know him like you don't. You look through blind eyes and think you know more, but in reality boyfriends hide A LOT from their girlfriends and people outside the relationship will know about things like cheating far more than you would.
Don't be too quick to say you don't know him like I do. Because you don't know him like they do.
leave him if ya worried its not worth it