Why? Why doubt that?
What have the current crop of republicans done to show that they have any sort of integrity or morality? When they supported President "Grab 'em" Trump? When they saw children being locked in cages and said "That's good"? When they invoked a non-existent "Biden Rule" to prevent Obama from more supreme court appointments?
Yes, occasionally you will get a republican making some sort of empty statement criticizing Trump. But at the end of the day they all fall in line and do what they can to support the party in power. This is the party that approved the nomination of Pruitt DeVos, and Flynn. This is the party that crammed through a tax bill that drives up the deficit by billions, in which early drafts featured illegible text scribbled in the margins (that congress critters were expected to vote for).
The republicans are currently without morality, and the idea of them voting to confirm someone where actual evidence of assault exists should not be dismissed. I'm sure they would come up with some moral justification. "Give him a mulligan."
ETA: I have to think the republicans may really be worried. This may be their only chance to 'stack' the supreme court with right-wingers.
If Kavenaugh doesn't get confirmed, they probably don't have enough time to cram another nominee through the process before the midterms. And although the Democrats are at a disadvantage and the chance is small, its not impossible for them to pick up a couple of seats in the senate and form a majority. That will limit the ability of Trump to appoint any more right-wing judges. At best, Trump will have to nominate more centerest judges. At worst, we'll get some gridlock for a few years. I suspect the republicans are considering that when they decide to confirm Kavenaugh.
The legal system often has to deal with uncertainties, and in situations like this instead of dealing with absolutes, you may have to go with a preponderance of evidence.