God loves everyone equally and there is no sin that He won't forgive. It must also be said, though, that God does have laws. Sin is the act of breaking these laws. Every time someone breaks one of these laws, God is willing to unconditionally forgive the offence; however, there are consequences of sin regardless of God's infinite capacity to forgive.
One of the duties of the divinely established, catholic, and apostolic Church is to regulate God's laws on earth in His name and authority. This authority was given to Peter in Matthew 16:19 and to the other Apostles in Matthew 18:18: "Whatever you bind upon earth shall be bound in heaven, and whatever you loose upon earth shall be loosed in heaven." This authority is over all humans on earth whether they are in full union with the Catholic Church or not. How the Catholic Church regulates and enforces these divine laws depends on one's unity with the Church.
One such law is that sexual intercourse is only permitted between a man and a woman that have been joined in a sacramental marriage, which is impossible to terminate as long as both spouses are still alive. If one of the spouses dies, the other is free to marry again.
The reason why sexual intercourse is held so sacred by God is because it reflects His image (cf. Genesis 1:27). When a married couple perform the marriage act, they are in the very image of God. It is no mere coincidence that a man and women can procreate when they are in the image of God. This is a direct reflection of God ability to create. As well, a sacramental marriage cannot be terminated anymore than God Himself can be terminated.
This is why marriage is a sacrament, and not just one of seven sacraments, but the archetype of all the sacraments. This may surprise some. Even the Eucharist is based on marriage. Marriage was established right at the creation of man, and foreshadowed the Eucharist. The Eucharist is God, and marriage is the image of God.
A crime against marriage is a denial of the true identity of God, and is as serious as a crime against the Eucharist. If one has committed a crime against marriage with full knowledge and deliberate consent, he cannot receive the Eucharist without first reconciling with God in the Sacrament of Reconciliation. In fact, receiving the Eucharist after committing a crime against marriage and before receiving absolution is a serious crime against the Eucharist (cf. 1 Corinthians 11:27). First you physically deny the identity of God, and then you physically receive God. In essence, you are calling a curse upon yourself.
It would be very irresponsible for the Church to knowingly allow one of her children to call a curse upon himself. If an unmarried couple are living together, it is the responsibility of the Church to inform them that they cannot receive Communion. If a Catholic has not conformed to the specific rules for Catholics regarding marriage, such as getting married outside the Church without special permission, he also cannot receive Communion.
This is not a punishment or judgement. This is protection. Until a couple have their marriage convalidated in the Church, they cannot receive Communion.
Having a marriage convalidated is often referred to as having a marriage blessed in the Church, but this is incorrect. Convalidation takes an existing but irregular union and makes it into a valid marriage. A couple my feel they are married in the eyes of God, but if at least one of the partners is Catholic, he is bound by Catholic laws; therefore, if these laws are not obeyed, they are not married in the eyes of God. What the Church binds on earth is bound in heaven, and what the Church looses on earth is loosed in heaven. How the couple feels does not manipulate God.
Divorce is not a sin. A divorced person can still receive Communion. Civilly a couple can divorce; however, the Church does not recognise divorce and still considers them married (cf. Matthew 5:31,32; Matthew 19:9; Mark 10:11,12); therefore, they cannot re-marry.
The validity of a marriage can be questioned. If it can be determined that a marriage was invalid, the couple can attempt marriage again. The determination that a marriage was invalid is called an annulment. A divorced person cannot attempt marriage again without an annulment because that person may already be validly married. Therefore, to attempt marriage again would be a crime against the image of God.
Usually, when a civil divorce takes place, the underlying reasons for that divorce invalidate the marriage. Therefore, an annulment would likely be granted. This does not mean that all petitions for annulments are granted. In such a serious matter, the Church must be certain that a marriage was indeed invalid before granting an annulment.
Marriages of non-Catholics are always assumed to be valid and both partners are are validly baptised, sacramental; therefore, if a divorced person that attempts marriage again wants to come into full union with the Catholic Church, the invalidity of his first marriage must be established before he can be admitted to the sacraments. This may appear unwelcoming, but it is for that person's own protection. Technically the person can be received into the Church; however, he cannot receive the sacraments, so there really is no point. It would be like inviting a person to supper but not allowing them to eat anything.
A similar scenario that often appears unwelcoming is if a Catholic marries in the Church, divorces, and marries again without an annulment outside the Church to a non-Catholic who eventually decides to become Catholic. Obviously the Catholic partner cannot receive Communion until he receives an annulment and his marriage is convalidated. In addition to this, the non-Catholic partner cannot be admitted to the sacraments until her partner receives an annulment and their marriage is convalidated. Again, this person could be received into the Church; however, she would not be allowed to receive the sacraments, so there is no point. This is for her safety.
This is very confusing to many, and unfortunately appears very unwelcoming, but it is entirely for the protection of the individual. Marriage is very serious business; therefore, the Church must be very careful.