The Catholic Church treats many as being of second class just because of the kind of marriage they have

A Catechism of Christian Doctrine (CTS, London, 1978) used to say that marriages between Catholics and other religions were dangerous and only tolerated for grave reasons (Question 310). The present version still says that the Church does not encourage mixed marriages and has dropped the insulting bit. Sometimes, Catholics will say that they are also opposed to mixed marriages because the partners being of two different religions makes things difficult. That is a denial that marriage is a sacrament – a means of grace and that marriage is difficult anyway which is why it needs to be a sacrament.
 
Catholics will believe that if the partner who is not a Catholic will not convert it is because he is not examining the Catholic faith properly nor is he allowing God to work in him so he is a mortal sinner. The Church claims to be the truth just as Christ was the truth and anybody then who doesn't see the truth is not concerned enough about it and so is a sinner. The Church expresses its view by describing people who are not under the papal regime as non-Catholics. You don't speak of the world apart from Buddhist as non-Buddhist unless you think that everybody should be Buddhist but is not. And so the Church speaks of non-Catholics meaning that anybody who is not a Catholic is defective or falling short in some way.
 
Roman Catholicism says it cannot let a person who is suspected of mortal sin marry in that state for it defiles the sacrament. Also, when you reject God you reject all goodness – and the love of God and of neighbour go together in religion - and so you cannot really intend to do the good thing of forming a union of holiness or love with your partner. The marriages of sinners are invalid. This shows that probably all marriages can be annulled.
 
A consistent religion cannot allow mixed marriages at all for the partner of a different religion who makes no attempt to convert you is betraying and sinning against her or his own belief or religion.
 
If the unbelieving partner refuses to try and convert his Roman Catholic wife or gives us, he is refusing to let her see what he believes to be the truth. This would be mortal sin and not love and love is supposed to be the basis of marriage. Mixed marriages cannot be countenanced by the true Catholic under any circumstances.
 
Christian marriage is made for the sake of God. God binds the pair together. It is not binding at all if God does not do this. The law cannot really marry you. God can’t bind in sinful marriage.
 
Rome lets good Catholics marry heretical Catholics which is not fair when it discourages marriage with Protestants.
 
Rome allows divorce if you are baptised and your husband or wife is not. Though the marriage was real, the Church can dissolve it and allow you to marry again. This rule will wreak havoc in the future for there will be more unbaptised people marrying Catholics than there are currently.
 
If mixed marriages are bad then it is obviously a sin to be good friends with anybody who does not belong to your religion. It is taking the risk of having your faith corrupted and when faith is of supreme importance in religion it is wrong to take a step closer to parting with it. Moreover, it would be equally bad to associate with anybody who claims to be a member of your religion who denies something it teaches or who refuses to obey it as far as possible. It is easier to commit sin when you are used to sinners and like them. Your friends are supposed to lead you to higher spiritual union with God in faith and service so they have to be from your own religion and determined to stay in and maintain that religion.

Such association reduces the religionist’s hatred for what he considers to be their errors and makes it harder for him or her to want to convert them. It contradicts the fact that if God exists and is love then he comes first.
 
If you should not marry a person who doesn't have the same faith as you or who doesn't take it seriously if they do, then how can it be wrong if you refuse to employ them for that reason? Marrying is a bigger thing than hiring.

A religion worthy of the name or sincere will make members marry only inside itself, have friends who adhere to it loyally and devotedly and employ only those who belong to it.