My parents don't want me to be with my boyfriend, help

I love my parents and I'm close with them, but I also love my boyfriend so much so I'm really conflicted. We've been together since we were 15, he's really sweet to me and always appreciates me/loves me/etc. I feel really really strongly about him, I've never felt this way about someone and we've been together quite a while now but my parents really disapprove. 

The tough thing is that what they view as disrespect isn't anything crazy- he always has me home on time, treats me well (although they always tell me I deserve better), etc. He doesn't smoke, do drugs, etc. But he does differ in manners from the way I was brought up- he does open doors for me, still pays for dinner after all this time, drives hours for me and things like that, but when he stays over in our guest room he normally doesn't make the bed and my parents told me he left a Coke bottle and towel on the floor and that they were wondering if he ever learned any manners. 

He really has not done anything to have my parents dislike him this much, but they always say he's not right for me (but they won't even give him a chance or get to know him the way I do). He tries to have a good relationship with my parents but they're kind of just cold to him. All my friends love him, so it sucks to not have my parents support as well. My mom even goes so far as to literally point out other guys she thinks I should date and it really upsets me. I tell her that she knows I have a boyfriend and she has literally said to me "yeah well it won't last". 

I have a feeling our relationship is going to be long-term, and I don't want to put up with this anymore. It's not like he's some bad boy awful influence, he's  going to college and doing the ROTC program and eventually wants to be a marine lawyer. 

I don't know if maybe my parents are right and I'm just blind because of how much I love him, but I doubt it. They just won't give him a chance. 

Any advice?