I also agree, that the degree that Trump has normalized previously unacceptable behavior for president is astounding. I'm hoping that historically this will be an abberration rather than some kind of turning point for both the presidency and our country. As far as what was acceptable in the past, and boys will be boys, you have to remember that Warren G Harding's reputation for be considered the worst or one of the worse presidents, in addition to corruption in his administration and being considered a weak president, was that solid evidence of his affairs came to light. Previously there were rumors, but it was only when it was evidence it changed people's view.
Going forward, there seems to be sufficient evidence that Trump has violated Federal laws. The one with the most evidence is election laws, that foreign officials cannot give anything of value for elections. Trump publically solicited the Russians to hack into servers of who was a sitting sec of state (another crime) his team including his own son met with representatives to discuss this exchange. We know that a number of Hillary's emails were released in an attempt to sway the election. This is a serious crime. I would not be surprised if we uncovered more evidence of gifts from foreign officials, in particular for quid pro quo.
While Trump fulfills the spirit of the law of treason, if one interprets strictly that we have to be at war with the country, does not fulfill the letter of that law. However there is a related law called seditious conspiracy, that he can be tried under, that what we know as of this moment, he is guilty of.
The most straightforward way to get him impeached is simply to have him testify under oath, about, really almost anything. He has lied about so many things, that are so much more important than consensual sex with an intern. Off the top of my head whether he knew about the meeting about Hillary's emails, why won't he release his tax returns or what is in his tax returns, what kinds of discussions happened when he visited Russia in 2013, or even what was discussed with Putin at his most recent private meeting? If we had a non-corrupt congress, this would have already happened. None of these things can be considered trivial, or personal matters. Rather they strike at the very heart of us having a functioning, real democracy.