We don't know if sexual harassment was an impeachable offense because no one tried. There are no lists of what is or is not an impeachable offense, congress makes that decision. I am not sure they would have voted that way, probably not, but the question we should determine is should it be?
I don't rightly know. But the point is that, while there's some question whether presidents can be impeached for having a consensual sexual relationship with a subordinate, there's little question that the charges levied against him very much were impeachable offenses.
Bill Clinton didn't remain in office because the Republicans screwed up. He remained in office because Congressional Democrats weren't willing to bear the likely political costs of sacrificing one of their own -- especially by siding with the execrable Gingrich & Co.
And it's folly to say that times have changed. If the same events occurred today, I'm confident it would end similarly. The stakes are, rightly or wrongly, perceived as being too high.