Firstly, "the left" never said that.
Collectively? No. But that attitude is so obvious as to be undeniable, really.
It said that the realm of politics and the realm of religion are separate and should remain separate. The government shouldn't involve itself with religion, and religious institutions (e.g: the catholic church) shouldn't involve itself with politics.
That is a standard at odds with historical practice in America.
Secondly, it would seem to me that the issue is practicing what you preach. For some 38 years now, a certain subset of Christianity has made itself out to be the morality police, railing against whatever "immorality" they see.
And in the end, what did it get them? The country did not become more conservative over the past 38 years, even though it plausibly could have. Their own elected officials threw every fight the voters hoped they would wage. These people never forgot to give tax cuts to billionaires but the issues that the average Republican Party voter cared about was largely ignored.
They were (rightly) fierce about Clinton's many improprieties in the '90s,
All of which were defended (or ignored) by Feminism, Inc. And frankly, it arguably didn't ruin President Clinton's leadership. To the degree polls can be trusted, they strongly indicated that he would have handily won the 2000 election if he'd been eligible to campaign.
they excuse Trump's crotch grabbing
I realize we can't quote Trump verbatim here. But read his quote. He never admitted to doing that himself. Was he ever in a position to have done it? Maybe so. But his quote is hardly an admission of anything.
and Stormy Daniels scandals.
Trump never campaigned as a moral authority. He campaigned as somebody who would be a good CEO for America. Voters knew what they were getting with him. This Daniels thing would've been harmful to George W. Bush or a Mitt Romney or probably even a John McCain. But it isn't harming Trump because most people figured he's got stuff like that in his history.
They have railed against cursing and lewd speech in public,
And they were shouted down.
Those same people are quiet as church mice when it comes to Trump's boorish, sexist, racist speech.
They haven't heard Trump say anything they find especially objectionable. You may find some of his statements off-putting but you (and others) really have no business attempting to dictate what other people should find offensive. If the culture war American went through established nothing else, it's that not everybody finds the same things offensive, yes?
If you don't even live up to your own standards, what good is it to have those standards in the first place?
Different factions of the right support Trump for different reasons. But I look at the GOP voters between the ages of 40-65 who supported the Republican option in most Presidential elections of the past and I see a very shrewd calculus taking place. In their view, the only side that's expected to be Mr. Nice Guy and "play by the rules" is theirs. They believe the left is able to get away with anything and will always have friendly news media and a compliant political establishment to make their excuses for them. They then ask themselves why they're bothering with a shiny, happy people public face when mostly all it gets them is defeat after defeat.
It doesn't matter if any of that is factually true. It doesn't matter that some people might disagree with them very strongly about all those things. The first, last and only thing that matters is
that's how they see things they voted accordingly.
I look at the Republican voters under the age of 30... well, maybe that's a subject for another day.
As to the other, from the outside looking in, the message I'm hearing from the left is they're mad that their usual smears don't work anymore. As above, it doesn't matter what the truth actually is.
That's how it looks. The left is accustomed to Literally Hitler™, sexist/misogynist and homophobe being enough to ensure a compliant (and, ideally, defeated) Republican candidate.
Those smears don't work on Trump and they haven't been working on his supporters for quite some time now. And whether the left intends to communicate this message or not, the vibe I'm getting is they're throwing tantrums that they can't engage in character assassination as effectively as they used to be able to.