A few thoughts for consideration.
Lust is a sin. Jesus said, " But I say to you, that whosoever shall look on a woman to
lust after her, hath already committed adultery with her in his heart."
You will not find a single passage in scripture which uses the term lust in anyway but the most negative terms. It's not acceptable. It's degrading and devoid of virtue. You may be attracted to people but when your attraction turns selfish it becomes lust. Doesn't really matter whether the object of your lust (object being the key word) is your spouse or a stranger, it's still sinful.
Catholic teaching on adultery....
Adultery
2380 Adultery refers to marital infidelity. When two partners, of whom at least one is married to another party, have sexual relations - even transient ones - they commit adultery. Christ condemns even adultery of mere desire.171 The sixth commandment and the New Testament forbid adultery absolutely.172 The prophets denounce the gravity of adultery; they see it as an image of the sin of idolatry.173 2381 Adultery is an injustice. He who commits adultery fails in his commitment. He does injury to the sign of the covenant which the marriage bond is, transgresses the rights of the other spouse, and undermines the institution of marriage by breaking the contract on which it is based. He compromises the good of human generation and the welfare of children who need their parents' stable union
So sex with yourself is basically adultery. It's marital infidelity.
It's also contrary to chastity as noted:
2396 Among the sins gravely contrary to chastity are masturbation, fornication, pornography, and homosexual practices.
As for Onanism, it's faulty to disregard scripture to suit a personal desire. Up until the 1900's every Christian and Jewish tradition fully understood that Onan commit a grave sin deserving of death by "spilling his seed on the ground". There are plenty of great studies on this. A few quick links:
http://www.rtforum.org/lt/lt67.html
http://www.catholic.com/thisrock/1991/9107chap.asp
The bottom line on Onan is this, there was a prescribed punishment for breaking levirate law. It was public humiliation. Death was reserved by God's law for sins that destroyed life/pro-creation.
There is no logical basis to believe that God killed Onan for disobeying the law and if you read the above mentioned links there is good reason to believe that God considers it a grave sin.
One way that I've described the disordered nature of masturbation to my youth group is this.
God gave us our senses to assist our intelligence in understanding Him more fully and for living our life here on earth in the span of time.
For instance, he gave us the sense of taste. If we didn't have the sense of taste, would we eat? Would we no the difference between good food and bad food. God knows that we need to eat to live. He graciously gave us a sense of taste to allow us to enjoy eating and thereby ensure that we will continue to eat to live. This sense of taste however can be misused. When we begin to eat simply because we like the taste, we have disordered our senses. The pleasure of taste becomes the focus rather than the efficacy of eating to live.
The same holds true with sex. God gave us intercourse to procreate and to give us an example or means of simply experiencing the unity that He has as the Trinitarian unification. Man and woman become one. He graciously gave us a sense of pleasure to allow us to enjoy intercourse and thereby ensure that will will continue to procreate and experience the unification of marriage. When we begin to find was to experience the pleasure of sex without the unifying and procreative means of intercourse, we have disordered our senses. The pleasure becomes the focus rather than the unification and procreation.
And honestly, if someone could please show me a single instance of a dissatisfied man or woman who had refrained from all sexual experiences prior to marriage. If you know nothing about sex prior to marital relations that how can you be dissatisfied? Or have you fabricated something in your mind based on tv, the movies, books, etc. that insinuate a certain level of pleasure and satisfaction? The dissatisfaction that is currently studied is not of virginal respondents but of men and women who have already disordered their senses. Step outside of societies pummelling of our senses. Get a Godly view and live in a chaste manner and you will find that there is a greater pleasure given in marriage.
Each person will make his/her own decision about masturbation but it is not wise to disregard, ignore or refuse to hear what the Church teaches in this regard. It is a grave sin. It will not lead to a better life with spouse or eternally.
There is a vast amount of writings on the connection between the marriage covenant and the Divine covenant. On sexuality within the context of marriage. On ordered love vs disordered love. As has been recommended already, theology of the body, humanae vitae, and many other Catholic sources.
It would be well invested time to pray and read to understand the wholeness of Church teaching before disregarding it as archaic based on message board responses.
Lean on your own understanding at your own risk.
