Have you ever been told by your friends that your new love interest isn't the best looking? Well if that's the case then tell them politely to shove it because true beauty is in the eye of the beholder. There is no way you should let the friends in your life not be supportive of your new boyfriend or girlfriend, and if they are not being supportive of you then they probably aren't the best friends you can have anyway so it may not be even worth sticking with them as friends.
Beauty isn't everything in attraction but you have to physically attracted to someone to be with them, beauty is definitely in the eye of the beholder as you are the only one who has to go out with the other person your friends certainly won't be dating him so what does it matter what they think. If they don't fancy him just let them get to know him as they don't have to be attracted to him physically and if they think you can do better for yourself, don't listen not if they are talking in terms of physical attractiveness anyway. You need to make sure you are happy with everything and if you are then you can tell your friends where to go, and they will eventually be supportive.
Beauty is definitely in the eye of the beholder as you are the initial one who met your partner, and you were satisfied enough to make them your partner so again what does what your friends think matter, not for a lot really as it shouldn't have an effect on what you think towards your relationship and it definitely should not make you consider your relationship.
The chances are at some point one of your partners friends would have said something similar to what your friends had said, as you cannot be everybody's type and it is more than likely that you are not attractive to all his friends so don't be put off if you hear something like that. Bottom line only listen to your own mind.