Do you think it's wrong that I'm ashamed of my boyfriend for being overweight?
It's not very nice, have you talked to your boyfriend about your feelings?
I did and he seems fine with the fact that I'm ashamed of him.
You need to examine why you're feeling these feelings about his weight. This could be an issue in your relationship and it could cause a rift between the two of you. You should accept him for who he is.
I would prefer to date someone thinner.
If you love your boyfriend, you should love him for who he is and his look shouldn't matter. You shouldn't judge him like that and accept for who he is.