Why must he go? Because he did a bad personal thing? So what. If his politics are sound, keep him. If not, that’s a reason to fire him.
You are missing the point. What if some nefarious people got a hold of this and blackmailed him? What if that was already done. These things compromise our congressmen. This is the stuff Hollywood blockbusters are made of. It also means that he is a LIAR. If he lies about this what else would he lie about?