Grinding is a dance people do for fun. Most of the time it doesn't mean anything. If someone does something to make you uncomfortable you have every right to stop them.
Grinding your teeth can be bad as it will wear down your teeth and may cause you to have pain and problems in your jaw.
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I think to some looking at the couple who is grind dancing from afar it may appear you are having sex with your clothes on (if that is at all possible). I think for the person doing the grind dancing if you are both willing subjects and you both get something out of it, she gets wet down below and he gets a huge boner, then yeah go for it. You will remember those experiences for the rest of your life especially if this is the first time. If from all the grinding one or both of you ejaculates then think of it as a bonus because it is possible.