If you don't want to go, don't go, say no, is it so hard to say? You should have your own opinion. If your friends really want to exercise, they won't go because there are fewer people who don't offer discounts, and they won't force you to accompany them because there are more people who can offer discounts. You'd better stick to your point of view and don't let them think that you can go anywhere, so your feelings will be very dangerous.