I have to first say that I doubt you and I would be in complete agreement as to what Trump has lied about.
But ignoring that...every lie...and I mean every lie...bothers me. But there are two things about this one that make it a bit more troublesome for me:
1. The number of times he has restated it.
2. The fact that Clinton was impeached for lying about a somewhat similar situation (including admittedly via a sworn deposition, but still...)