[Itse]

Hmm, there's actually a lot of questions in the OP I think I can say something to...

Is this nothing more than a religiously motivated problem...

Religion obviously plays a part here, but I don't think the concerns over these issues are something only religious people have. (Atheists can be conservative prudes too )

...or will our increasing sexual liberty prove to be the downfall of our society?

Only if you define "our society" as "society as we know it". Societies change and people change over time. Societies will never be destroyed as long as there are people. Personally I believe sexual liberation is generally a good trend simply because it makes it easier for people to fit their lives to their unique situations without the judgment of others.

I believe each relationship is unique, and trying to treat them as if they were the same only leads to trouble.

If there is something bad that religions bring to the equation it is the idea of a monolithic, uniform type of relationship that should work for every couple and every committed relationship everywhere and in every situation. This is sometimes called marriage.

(I put some personal history behind the tags since some might be interested in them and some might find them distracting.)

Spoiler!

I am now married for the second time in my life, and I think a big reason why this marriage lasts and the first one didn't is because at a time of crisis we finally understood that marriage in itself is just a word. All the rest is something we have to figure out for ourselves. We needed to throw out the stereotypes. In one part this also meant that we needed to find out what fidelity and trust really mean to us. Among other things it lead to greater sexual liberties for both of us. Which for people who need to also earn a living, raise a kid, clean the house and all the rest of life propably isn't half as exciting as it sounds to outsiders (As for sex, since someone is always interested, for me it means a couple of extra times of sex per year. So not exactly a big deal in itself.)

That was kind of a long way to explain it, but I think the whole problem with the discussion of "sexual liberty" is that it's so damned adolescent, as if relationships were mostly about sex.

For example I have a very strong, long-term and in part romantic relationship with someone I never have sex. (Long story and I'm not sure there is a clear explanation, but right now I think basicly we've just gotten used to things being the way they are.) However it started long before I even met my current wife, and the lack of sex in that relationships did not stop it from bothering my wife. Not the only reason for our crisis, but related to it. Right now my wife barely remembers that there ever was a problem, even though the relationship is propably more intimate than before.

Hence polyamory, not polysexuality. And no, we don't "swing", the whole idea is really weird to me.

But yes, for us it is kind of like having our cake and eating it; we can enjoy living in a long-term happy relationship while being open to meeting other people. (With limits that are essentially figured out on a case-by-case basis. Yeah, kind of complicated, but I think trying to face life with rules set in stone will only have you bashing your head on that stone at some point.)

(BTW I avoid talking about my wife's situation because she would not like that.)

Also, historically speaking sexual liberation is not exactly an evergoing trend. There have always been more conservative and more liberal times following each other.

Is lust merely a benign fact of biology, or is it indistinguishable with infidelity and adultery?

With humans lust is never simply about biology, but way more complicated, and very often deeply connected with the emotional and psychological needs of a person. This is why lust is not simply a factor of visual or physical stimulation, as most adults propably have at some point experienced.

As to adultery and infidelity, I would say lust has about as much to do with that as money has to do with fraud. Sometimes they are connected and sometimes they are not. The key word is trust and the betrayal of it.

Affairs often happen because of emotional needs or wants, and propably very rarely only because of physical needs or wants, although it can certainly be a part of the picture.