Can a man sleep with only 1 woman in his life? Relationship insecurities?

[......]

I have been with my boyfriend for 2 years, we got together when he was 19 and I was 21. He is just under 2 years younger than me. I found out that he hadn't done anything at all with any other girl but didn't ever think it would get serious so thought nothing of it. 2 years later and I love him with all my heart and I feel like, by being with him, I'm setting myself up for disappointment and heartbreak because I obviously want to spend my life with him and feel anxious and upset to feel as if I'm in a relationship that will inevitably end because he'll want to experience things before settling down.

People crave variety and even if he did stay, wouldn't that always be on his mind, even if he's lovely (which he is) and loves me, wouldnt he always be trying to vent those frustrations through the porn he watches (watching porn is fine by me) and the women he thinks about and the fantasies he has? Wouldn't he resent me?

I recently discovered he follows a girl on twitter who just randomly takes pictures of her huge DD breasts and he sits there clicking through them and drooling and while that's very insignificant generally, I'm flat chested, he's never had another girlfriend, so isnt that just another thing he hasnt experienced, boobs lol Its ridiculous.
It's like me being with one man, with an extremely small penis, while in the scheme of things thats not what a relationship or love is based on, wouldn't it always be 'what if'?

[M]

no

[Fred F]

I have for 51 years now.

Sure sometimes your mind wanders a little, but just don't follow through with the thought.

[ATP]

How is everything else in your relationship. If both of you believe that your relationship will last, then it will. A lot of people in different cultures marry their first love, so it is very possible for a man to sleep with only 1 woman his whole life. It's not for everyone in every situation, but I don't see why your boyfriend wouldn't stay with you.

[Staap It]

the grass is always greener over there. A good girl can keep a guy. Point this out stealthily. You have to make him think it is his thought. Maybe something like : You know I take better care of you then anyone else would, and your always happy and have fun too.

That is of course if it's true and he knows it.

A good girl is very hard to find, most girls just wear you out. Exhausting !
Others are just very nice to have around and always welcome to be around. It seems like a difficult thing to be for most girls. I know cause I know a good girl, it is like being with sweetness and she looks it.