Let me see if I can 'splain with a rough diagram.
This is not scientific, I'm simply trying to get across a general point.
The diagram (which I made up) represents people's dispositions about the opposite sex in general. The large majority of people fall in the middle, at the peak of the
orange Bell Curve. A small percentile have little or no tolerance and a small percentile of people are overtly and aggressively sexist.
Whether truly accurate or not, I think we can agree, attitudes, by-and large, form a Bell Curve. Most peopel ni themiddle, few people have extreme attitudes.
So:
davec has low tolerance,
pinball has slightly more tolerance,
Archie Bunker is a bigot.
Greepy McGrabberson is a menace.
The dots for each simply represent a range individual incidents for that person - a joke here, an innuendo, etc. Some are tame, some more spicy.
But look what happens if society overall becomes more tolerant. The orange curve pushes to the right, becoming the
red curve.
Creepy McGrabberson's worst behaviors (the right dots in his green cluster) now become more acceptable (in his view).
His sexual aggression -which was beyond tolerability before - is now within society's tolerance.
In fact,
the entire striped grey area is filled with people whose behavior was not tolerated in a society with low tolerance but now all those people feel that society will tolerate the transgressions that they couldn't enact before - or, at the least, they feel they can
get away with them. That's a LOT of people who are now emboldened by society's shifting guardrails.

(Notice, by the way, how this addresses your "gateway" theory (which is
not what we were proffering).
Individuals don't move around. Individuals stay where they are.
pinball may have a predilection for off-colour jokes, but
he does not slide across the chart - pinball does
not suddenly decide to rape someone. Off-colour jokes are not a "gateway" for pinball to change his behavior. The worst he might do - in a more tolerant society - is increase the incidence of things he's
already doing (his rightmost dots). Perhaps off-colour jokes are more acceptable in the office or some such. So, tolerance is not a
gateway; it is a
filter that opens wide.)