First and foremost, keep in mind that XMA does not equal gymnastics.  That is a whole seperate beast of its own.  Once you start talking about flipping, you have already lost them.  So let them know that they don't have to flip in order to do XMA.  This might help a bit.  Remember the vast majority of your adults are need to go to work the next day with as little "pain" as possible.

XMA like any other martial art program can benefit people of all ages, but people of all ages will not take to it in the same matter.  So with that said, how do you get those adults in there?  Why not break the classes into 2 seperate phases.  Phase 1 would be for more beginners.  People just starting up and trying it out.  That way you can go at a bit more slower pace which will get the adults a bit more confidence to go in there.  The Phase 2 class would be comprised of more advance people and would be at a faster pace than Phase 1.  So if they feel like they are in a little bit slower pace class this may help. 

Now have you done an XMA launch party?  If not you should.  I can give you some insite based on what we have done in the past.

My first thought on the adult situation is to have an "adults only" introduction.  That way, kids won't be around and they will feel a bit more confident in doing it.  Find a convienent time to schudule this class and then tell the adults that this will be an introductory class for adults only to see how XMA can help push their training to the next level.  Let them know that this will be a pre-registered event only.  This means they will need to sign up prior to the event.  This will give it a sense of importance and exclusiveness, while at the same time, give you a good idea on how many people will show up.  Also let them know that there needs to be a minimum of 5 people to sign up in order to run this class.  This may help them sort of motivate each other to sign up. 

The next thing is to sort of individually go up to a couple of them and let them know "I really hope you can make this class on the XMA, you seem to have a knack on picking things up and I bet you would really get a kick out of this"  In other words find a strong point each adult has and really push how that strong point can be an asset to learning XMA.  My guess is they will still object with a weak point that they have.  This is perfect, because it gives you a chance to sell the benefits of the program to them on a personal basis instead of a general speech given to the class.  Once you do the class you need to hype it up the next week during the regular classes.  People will be asking how it went and you need to be "It was awesome, we had a great time.  I was really impressed by how John Doe took to it and nailed some of these techniques. It was too bad you weren't there.  You would have had a blast.  Tell you what, we had a lot of people interested that had prior plans so we are thinking of holding another one, you should definetly come out." Now this may lead them to throw up their objections, which, once again, gives you a chance to push the benefits on a personal basis. 

As for an XMA launch party, this, in my opinoin is one of the most crucial steps in getting people interested.  Here is the jist of it.  You set a date to do an XMA party and demo, I recommend a 3 weeks out.  You hype that thing to death almost a "If you miss it, the world as we know it will come to and end".  Be sure to let them know that people will be able to participate in this event so wear something that they can work out in. 

When the day comes you have a few of your XMA people do a demo starting with the person who has been in it the shortest amount of time and then lead up to those who have been in it the longest, be sure to end the demo with you key players that is what gets people really psyched up about it.  Once they are excited right after the demo you get them on the floor and you run basic drills with them.  Have some high energy (KID FRIENDLY) music playing to help keep the energy level up.  When all is said and done let them know that if they sign up for the program on that day you will give them a awesome discount.  If they decide to wait one week, you will still offer a nice discount, then give them a deadline to receive an ok discount. The key to all of this is the excitment and energy that you put out to the group.  It may sound easy, but trust me, it takes a lot of stamina to keep yourself at the peak of excitement  while hyping other people up too....and you do need to keep them hyped, like a motivation speaker.

I hope this helps.