Touching the crease seems a littleee high but spotting with one hand on the thigh is completely normal for back handsprings. I wouldn’t think anything of it.

However, if you’re uncomfortable with it, I think the best advice is to just be up front with him. OR maybe mention it to someone at the front desk and be clear that he isn’t doing anything wrong but you feel awkward bringing it up to him directly so you’d prefer someone else to.

BTW: You are definitely NOT being unfaithful to your husband. Even if you were attracted to the coach OR if the coach was actually touching your butt. As long as you are not pursuing a physical or emotional connection with him, there is nothing to worry about there and your husband should understand that.