Generally, when a massage therapist works on the gluteus, they would expose the area (except privates). They can expose only a portion or the whole, should I say, butt- but these are all part and parcel of a routine. Nothing more to it. There are spas, massage clinics or wellness centers that does not include any massage on that area. If you are uncomfortable being touched in any area of your body just say so and if you are also experiencing any sexual "techniques" and overtones (which sould not be the case) then do not exaggerate the sensual feeling because it might also cause an awkward feeling on the side of the therapist.
As much as possible, a massage and spa session should be kept within professional boundaries. A wholesome, healthy and relaxing experience.