There is a less appealing but also simple answer: they don't believe the victims.
Sure, it was reported months ago that Trump is a creep who just grabs and fondles women. But that was only a woman who said that. This, OTOH, is straight from his mouth so we can believe it.
Interesting interview with 5 Trump biographers. Should dispel the "locker room talk" argument.
The biographers basically say this is who Trump is - it's all about acquisition, by force if necessary.
http://www.politico.com/magazine/story/2016/10/donald-trump-2016-biographers-214350
I have a hard time relating to the average voter sometimes. It seems they are obviously interested in things I find to be of no consequence. For example, the thing I find so offensive in the tape is what slyjoe above referenced. It's the "If I want something I can just grab it" attitude, even if that thing that they want is a rather personal part of another person.
The fact that he is a sexual predator is of less consequence for me. He's every other sort of predator. Of course he's a sexual predator.
Other people must be upset at the vulgarity, which also isn't new on this tape, or the infidelity, but we already knew about that. Or maybe it is the pursuit of married women, but to me I don't care about that. They can say yes or no as that suits them, and it becomes an issue for them and their husbands.
But for me, that feeling of entitlement, of just plain ego. The lack of self control. The "I can't help myself, I just start kissing." Ok, fine, but now you want to have the power to decide who lives and dies? No. I want someone who has a little bit more control.
(And when it comes to the specifically sexual aspects of that tape, the most revolting part for me was actually the "give Donald a hug" part. Because of what those people were saying moments before, it was just way, way, creepy.)