The thing that kills me about this whole Trump thing is that he was never a very compelling con man. He's not a smooth talker. He's not an effective liar.
His appeal is that he says the things that a lot of people on forums like this have always wanted their candidates to say. It's the same appeal that the likes of Sarah Palin and other politicians have had (including Jeff Sessions and, at times, Ted Cruz); the same sort of appeal for some, that Ross Perot or Pat Buchanan have had.
That sort of appeal has its roots in the desire to hit back against a smug and arrogant foe that never seems to lose, or to stay defeated when they do lose a battle. In a guy like Trump, they see ... well, they see a bigger bully who looks like he might have a chance to win a fight.
The fact that Trump is a narcissistic grifter and a demagogue who appears to be using that appeal to his own ends -- none of that matters. He's still the guy who they think will hit back, and to them that's more important than any petty groping he may have done.
Pretty uncharitable words on my part. But to those who ardently support Trump, I think it's probably the explanation.