I have to disagree with Hanitaa.
Ask yourself this, if you went to a bachelorette party and there was some half-naked guy thrusting himself at you, would you feel comfortable touching and telling your boyfriend about this later? If you wouldn't, it's cheating. And if you would, than your relationship may not be as serious as you think.
No matter what the situation is, whether it's a bachelor party or not, there's no reason for a guy to actually touch. Yes, it's ok to watch, as we cannot always know what our eyes will see ahead of us, but actually making the move with our hands (or any other body part) is clearly a conscious decision. That is cheating.