"I've read within the Catholic apologia that the "Law" trumps compassion."
I've never seen that one. When did you got it? We are teached to always stay open to sinners, "hate the sin, not the sinner" and not judging, lest we are judged in turn. Compassion goes first. In fact, the whole point of Jesus is about how we came to deliver us from our due punishment, by dying. By Church dogma, the only sin that can not be forgiven is the belief that God will not forgive you if you are truly repentant.
"IOW, the sin of unlicensed sex should be punished by any and all available consequences."
Unlicensed sex is, by catholic belief, bad by itself. We believe that going around having sex carelessly it is demeaning for both parties and, sadly, brings consequences such as unwanted pregnacies and STDs (what does it means IOW? "in our own words"? sorry,I'm not a native english speaker)
"Advocating abstinence is quite ludicrous in cultures where sex is a form of entertainment."
That's why we believe it has been cheapened. Sex is suposed to be more important than just any other form of entretainment. We are sad that we live in a society when it has been reduced to just that.
"As a Gnostic, I say that Compassion is the only law... "
We believe the same. Deus Charitas Est.God is love and mercy above everything else.
"and I say further, any mortal who claims spiritual authority over others is a fucking liar."
We recognize the spiritual authority of the clergy on the base that they have studied and meditated more on God's nature and plan than the average believer, since they have dedicated their lives to doing so. That is also the reason for Church Tradition. We simply do not want to change what it works unless we have a reason for it.
A fun fact. Do you know how catholics define Hell? is not a lake of fire and brimstone. It is simply a permanent separation from God. C.S. Lewis said it best. Eventually, you say to God "Thy will be done" or God says to you "thy will be done". Going to Hell is strictly a matter of choice and an action between Creator and Created. That's why the fundies that go around yelling "you're gonna burn" piss me off so much. They have absolutely no way of knowing if someone is in Hell or not. We also do not know, but at least, we don't claim knowing.