First, my SO always comes first. If she isn't interested in something, then we find something else that we both can enjoy. This includes everything, even sex. If she wanted to stop swinging, then we stop. I would never think about leaving her (and that sounds like what you are implying) for greener grass...heck, I can't imaging grass any greener than her. Sex is fun because of who you are with, not who you are with is fun because of the sex.
Second, if you can't be honest with each other, then there are bigger problems. A great relationship REQUIRES love, trust, and honesty. If you don't have an abundance of all three, then there is where you start working. You will find once one begins to increase, they usually bring the other two along as well. Work on opening up your communication so you can both talk about everything. So she isn't interested (now) in having sex with other people. What IS she interested in? What are her fantasies? What can you do to make them come true? Maybe some day she could be interested in sex with others, but right now she doesn't feel confident and comfortable enough in the relationship to think about that (if a relationship is rock solid, asking to swing is like saying you want to have sex with others because she isn't good enough or you are looking for the NEXT relationship after her). Cart before the horse: make the relationship great and then see what you both are interested in pursuing.
As communication improves, so will the trust (and the love just comes along for the ride). Make sure she knows that you will not judge her or hold anything she says against her. You also need to be willing to open up yourself as well. She needs to know the same things: don't judge or hold things against you. It will take time and is a slow process, but she is worth it. Invest the time and see what happens. Encouraging her to dress sexier is a good start. She needs to know that you love it and are also there to 'protect' her (until she gains the confidence). Remember, there's no rush: you both have the rest of your lives together to get to wherever you are going. One step at a time and don't rush her and you may be surprised where your path may take you. Good luck and let us know how things progress.