My Boyfriend is Too 'Small' to Marry; How to Break-off Relations?

I am a 20-year-old young woman. I met a guy in May of this year. At first, I wasn't sure if I should've pursued a relationship with him because he was rough around the edges.

However, after a while he changed and became almost the perfect man.

There is only one issue: His penis is small and he doesn't satisfy me sexually. He enjoys giving me oral pleasure, and although that is okay, I still want to be satisfied the traditional way.

He regularly speaks about marriage and I should be excited. However, I am rather confused as to how to feel.

I don't want to leave him because he is a good man. But, on the other hand, I'm terrified of going into a marriage with a man who can't satisfy me because I will be unhappy.

I want to leave the relationship but I don't know how to do so, or on what grounds, because he treats me really well.

If I leave him, he will be devastated. Please advise me as to what to do.

What's your reaction?