No, what is a sin is to consider it to be important in any non-romantic relationship, or to consider it crucial even in a romantic relationship. People can’t help what they are physically attracted to, but they must override their reactions to appearance with ideas of the whole person based on reason and judgement.
I think that by and large, you shouldn’t marry someone you don’t find physically attractive. But at some point in the relationship (hopefully before marriage) love for the actual person starts taking over the load from mere physical attraction. This is good because the body and face will change over the years, but the person inside may only become even more beautiful, and that is something that you can help each other with.
Your premise is faulty and your conclusion does not follow from it.
No, everyone could not be thin if they just tried for a few (or even several) months. Even if they could, they may have (
gasp!) more important things to concentrate on in their lives. They may not have the energy for devoting themselves to a weight-loss/gain program because they are raising children, working hard, striving for spiritual growth, volunteering to help the needy, or any number of other things. It is not for you to decide what is most important in everyone else’s lives.
And aside from that, we are not to reject people based on their appearance. Not because they have funny teeth, or because they have a funny-shaped nose, or weird eyes or sticky-out ears (actually, sometimes that is kinda cute

), or because they are too fat or too thin or because they don’t wear the amount of makeup we happen to like.
Sometimes the way someone is dressed or made up is designed to make a statement, and it seems prudent to take that statement into account. But even so, we are not to reject people based on it.
You may feel that way, but the Church has never said so, and in my opinion, you have merely made a principle out of your idea that it is everyone’s responsibility not to offend your eyes by not being attractive. What about older people? Should they stay home because you don’t find them attractive? Or is it OK for elderly people to come into view, as long as they don’t commit the additional “sin” of being plump or skinny?
Your speech here, which is completely lacking in charity for those people who suffer from and fight a propensity to overweight or have a serious difficulty gaining weight, is not beautiful. If you do reject people for not looking the way you feel they ought to, your conduct in this matter is not beautiful either.
Please rethink your position in light of the fact that it really is what is inside that matters most. Being overweight is not like wearing smelly clothes, and even if it were, you really shouldn’t reject people for wearing them. It is only rude if it is done with the intent to offend. Otherwise it, like being overweight, is merely unfortunate.
–Jen