First, I would like to point out that there is a difference from being overweight and being unhealthy. I'm sure if we took a wrestler with some of those massive muscles and weighted them we'd find out that they weight more than they should, and it's not because they are fat. I'm sure that's not what you meant, but that's one thing that's sorta bugged me how weighing more than you should deems people as unhealthy when really it doesn't mean anything by itself.
I have been overweight all my life. When I was a kid my doctor would freak out cause I was off the chart for weight for my age. Of course being off the chart in height for my age didn't seem to factor into that though. Right now I'm borderline 5'10 so I've been told I'm suppose to weight 150 lbs. I hit 180 lbs for a while and thought I looked good. To go to 150 though I'd probably have to lose all the fat and muscle on my body, and I don't think I'd look good then.
That's one thing many need to understand is that you will not be attractive to EVERYONE. You may fit societies definition of sexy to a T but I'm sure there is someone out there that won't find it attractive. So, you're better off finding the shape that you find attractive (and that's healthy) and sticking with that, cause you will find someone that agrees with you. Or, if nothing else, someone might be attracted to you cause you are a confident person with your own definition of sexy.
I also agree with froggylover227 comment about how attractiveness changes. We're pretty much fighting an uphill battle with trying to be thin when we should try to be healthy. Someone could be a rail and still think they should be thinner. They might not be overweight, but it's still not healthy.
I have a friend who has lost probably 30 lbs since I met her and she looks good. Every time I hang out with her now some 40+ guy will come up to us and offer to buy us drinks. I'm sorry but if this is what I'm gonna have to deal with when I get back to my desired weight, I'd rather be overweight and ignored then the girl guys think will sleep with them if bought a few drinks.
I may joke about how I'd be the person people would pay to keep my clothes on, but I'm okay with how I look. If I lost weight I'd be happy, since I've gained about 20 lbs throughout my college years, but I'd also be fine if I stayed this way as well. What you really need to do is take a step back and figure out why you think less of yourself. Generally it's something someone said to you or because of an image you find attractive yet don't match.
I always look at it like this. Ever see that one guy that really doesn't fit the attractive definition but still thinks he's attractive. Or the Progressive commercial with Sonic and the guy who thinks he looks like the strongest man in the world. Strive to be more like him. Look in the mirror and see all the good things about yourself instead of the so-called "flaws." Wear what you feel sexy in instead of what you're told you should wear to feel sexy. Just be you cause you are the best you out there.