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

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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'?