Some of us would say that sex outside of marriage makes liars out of people; in fact, some of us would say that the act itself is a lie.
You wrote: "Some of us would say that sex outside of marriage makes liars out of people; in fact, some of us would say that the act itself is a lie."
And those who say that would be absolutely correct.
In fact, those who are married and who have lustful thoughts about a woman who walks by are adulterers, according to Jesus anyway, and have something to confess.
Nobody is without the crimson stain of sins, no, not one.
Fortunately, God is a forgiving and patient God.
Still, sanctimony about sex is Pharisaic. The lessons need to be practical:
Sex outside of marriage of whatever sort (including masturbation) and lustful thoughts within marriage, are all bad because they are sins and offend God. They must be confessed and atoned for. That is true.
And, sexual contact with others outside of marriage, although it is not MORALLY worse than masturbation or lustful thoughts, carries the addition risks of terminal and disfiguring diseases, and of unwanted pregnancies, which then carry the temptation to commit murder within them.
So, the best thing to do is to maintain a pure and holy mind and abstain from sex, including with yourself, or sexual thoughts, until marriage.
If you can't manage that, the second best thing to do is to commit the sin of lustful thoughts and masturbation, keep it to yourself and confess it to God, over and over and over again, if that's what it takes. Unrepented, it will send you to hell just like murder - it is an abomination - but at least you're just taking YOURSELF down and not taking anybody else down with you. And God is very, very forgiving, so long as you admit you're wrong and sinful and ASK him to forgive you.
But what happens if you fall prey to lust and you're not married. Well, then the third best thing to do is to be a monogamous fornicator and GET married to fix the problem. However, once you cross this threshold, not only is there the mortal sin (we crossed that threshold with lustful thoughts alone, or with reaching out to just our own persons), but there is the double-barrelled problem of taking OTHERS into mortal sin, AND risking illness AND risking pregnancy. The sin is the sin, and the single sexual sin is mortal, but once another person becomes intimately involved in it, the sins multiply and the potential opens up for physical damage in the here and now, and not only spiruital damage to the relationship with God which God will forgive if you ask him.
God forgives the soul of the masturbator and the serial sodomite fornicator, if either asks Him sincerely and contritely. BUT God doesn't clean out the body when He cleanses the soul, so the masturbator may be able to walk free of ill effects once he has forgiven, but the person with AIDS still has AIDS, and will still die of it. The sin is repented, but the flesh still is burnt by it and suffers.
Our MORAL disapprobation of sexual sin should not exceed that of God: it's all uniformly bad. But the PRACTICAL aspects of threats to life from sexual sins with others are much more dramatic than solitary sins.
Given the overwhelming pressure to sin that this thorn in the flesh, this sex drive with its enormous appetites places within us, there is spiritual damage, which is between us and God - and that's where it ends with solitary sins. Include others in the sin, whatever their gender or number, and the potential for expanding waves of harm and death become greater. There's no such thing as a MORE mortal sin. Mortal sin is mortal sin. One act of unforgiven masturbation will kill the soul just as dead as a lifetime as a serial homosexual prostitute. But the former can be expunged by forgiveness of God, as can the latter...but God doesn't kill the AIDS even when he forgives the sin.
The sin isn't worth with another, but the practical knock on effects in the physical world certainly are. We need to teach both consequences. We need clarity. And we must not lie to children and tell themnthings that are not true. It is not a WORSE sin to sleep with another than to play with yourself, and it is a sin, a MORTAL sin, to play with yourself. God is forgiving, but AIDS isn't, and neither is la lifetime of guilt over killing a child in the womb. God may well forgive, if you ask and mean it, but AIDS won't, and you won't forgive yourself either.
The honest truth is pretty bad as it is.
God is the nicest part of it, because HE will forgive.
AIDS won't.