By 'another very important advantage of having a long-term relationship is that boundaries disappear', I mean it's more of a mental thing.
Let's say that any reluctance disappeared.
With my wife's two regular lovers, she became a fully liberated woman. Or, having no reluctance at all.
This doesn't mean that there were strangers with whom she wasn't fully liberated. Fortunately.
It depends of course if there existed a certain level of chemistry. Whatever that might be.
From the outset, we didn't have any boundaries or rules in our lifestyle.
This was to avoid arguments about what could or couldn't.
An very clear example is French kissing.
If the male partner can't accept this, I think that he takes away a natural expression of being very satisfied.
Okay, you have to know that I never wasn't jealous at all.
So I can imagine that people have other principles. Quite usual I think.