I don't know, I just don't think anyone would find me sexy, as they never have
The thing is, sexual allure is incredibly personal. I know there are always newspaper articles etc about The World's Sexiest Women, but they will only ever appeal to a certain section of the opposite sex. For every man that finds, for example, Cameron Diaz sexy, there will be half a dozen more who think she's just a bit meh.
And it's not all down to pouty mouths and sticking-out bosoms, either. Take a look around you, at all the couples you see walking down the street. What made them get together? Are they all slim/toned/gorgeous? No. Of course they're not- except (one hopes) to the person they are with. It's an indefinable something- something that kind of 'calls' to another person.
What makes it last, of course, is an entirely different set of things- again, unique to each couple.
I would bet that people have found you sexy- but for one reason or another, you haven't got together with them. But that needn't always be the case.
It is all about confidence, really, as loads of others have mentioned. Timing plays its part too. But there's absolutely no reason why it shouldn't happen for you.