2 - Why does it matter? Because how can sex before marriage be a mortal sin if the Christians have only been practicing marrige for a short period of time? Did God change his mind?
Here you go. Part of this was my answer to Nolu's question. I pm it to her thinking no one else would care to know about it.. but since it also answers your question, maybe I should put it here, too.
If you understand what "sin" is (according to a relegious standpoint) it probably would have been a bit easier to understand why sex before marriage is consider sinful!
Sin is an offense against reason, truth, and right conscience; it is failure in genuine love for God and neighbor caused by a perverse attachment to certain goods. It wounds the nature of man and injures human solidarity. It has been defined as "an utterance, a deed, or a desire contrary to the eternal law".
Sin is an offense against God: "Against you, you alone, have I sinned, and done that which is evil in your sight." Sin sets itself against God's love for us and turns our hearts away from it. Like the first sin, it is disobedience, a revolt against God through the will to become "like gods," knowing and determining good and evil. Sin is thus "
love of oneself even to the contempt of God." In this proud self- exaltation, sin is diametrically opposed to the obedience of Jesus, which achieves our salvation.
There are two types of sin, mortal & venial. Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him. Venial sin allows charity to subsist, even though it offends and wounds it.
Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill,
Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother." The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.
One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent.
and that is why having sex before marriage is considered sinful (it violates God's 10 Commandments & goes against the Church teaching on the subject). The magnitude of the sin depends on whether one knows fully well that it is something he/she's not supposed to do, but choose to do it anyway. The blatant "hecks with God & His law" attitude will only make it worse.
God has given us the beautiful and wonderful gift of sex to be engaged in within the marriage union to make that marriage union unique within the experience of us all. Sexual intercourse within marriage is for this reason good and of great blessing both to the couple, to the family and to the church. However, outside of marriage, it's considered sinful, due to what I have explained above, it is not God's design or intend and therefore not His will.
Think about it. If your focus was not on sex, what is it that you (a very general "you", of course) would be looking for in a life partner? Sex tends to cloud that issue. We get all absorbed in it and it's such a wonderful feeling that we think sex is love and it isn't.