My lovely wife has the same fears, despite the fact that I find her as hot as the day we met (although way more skilled now ) she still has some fear that she won't be found sexy. The truth of the matter is she has never had trouble getting a playmate eager and willing, in large part because of her great attitude towards others, and the fun and friendly spirit she exudes. It all boils down to how you click with the other couple, more than any idea of physical perfection (what is that anyway?) as to how satisfied you will be with the encounter anyway.
I can almost guarantee that if you go to a meet and greet somewhere, dressed in a sexy get-up, and just talk with a few other couples, your confidence will go up, and nothing is sexier than confidence. Just meeting real people (who may or may not already be actively engaging in full swap) will help you find people that you are both comfortable with letting into your relationship to a greater or lesser extent.
If you go to a party/ M&G with the express intent of just getting to know a few people and get revved up for some of your own playtime later, you take away most of the fear of rejection and you can concentrate on having fun and finding others that like you for who you are, not what YOU think you look like. Plus any evening that winds up having some hot , fantasy fueled sex with your own wife is a win for both of you right?
Oddly enough, there are quite a few people in the lifestyle community who find thin people unattractive. What I'm saying is don't pre-judge your desirability because you two are exactly the couple some other couple is looking for. Warm, honest about what you want, and enthusiastic in the bedroom right? You'll do great. Come on, the water's fine!
Well, that was certainly long winded enough. I'll go back to