Should I break up with my boyfriend? Please help?

Question

I apoligize in advance for the length of my question. I just want to give all the details.
The history...My boyfriend and i have been together for 1 year and 7 months. Im 21 and he is 23. We've known eachother since i was 13 ( we live on the same road) and sort of dated when i was 16. Then i got into a car accident and didnt really hear from him. We both went or seperate ways. I dated my ex (i guess you can say first love) for a little over a year and he dated his ex for about 3 years. Then we ran into eachother 19 months ago and have been together since.
The problem... I find myself thinking about other guys. It started a little while back and i was able to shut it out. I felt so guilty because i was thinking about my ex. There are things that you just cannot forget about someone. Now i just find myself curious to whats out there. I know i could easily settle down with my boyfriend, start a family, buy a house and he would be a great husband/father and i would never have to think twice about him cheating or doing anything like that. I trust him with all my heart. But i find myself asking if i really want that right now? Im only 21 and i still want to hang out with friends and date. I feel if i let him go im letting go of a great person. And im sure ill regret it. But i feel like a horrible person for even feeling this way. The majority of the time we have a great time together and laugh. But when we argue and get annoyed with eachother, thats when my head starts spinning. Lastly, this never would have struck me to be a problem in the begining. I love to work out and stay in shape and my boyfriend well, is not in shape. He's done it before but ive noticed it alot more now.. he'll ask "so you like guys with six packs and a nice body huh?" i tell him " i like you just the way you are baby" and he'll get defensive and say ' just wait ill have a nice body' and so on... It drives me crazy. Or he tries to avoid movies that have guys with a six pack. COME ON! My problem solver is if it bothers you that much do something about it. When i told himthat he got very upset and convinced himself that i dont like his body. Please dont call me shallow because im not. This has just come up more than enough times.

The questions... What do i do? Has anyone ever been in this postion before? How do i deal with it? Any advice will do.