Here is a link to an article written by Jenny Uebbing over at Catholic News Agency describing why marriage can only be between a man and a woman.
It is worth your time to read.
I would just add that I find it unfortunate that in contemporary society we want to categorize people all the time. We are either Republican or Democrat, pro this or anti that. We do this partially for political purposes, so we can further polarization and conflict. I am talking about categorization, not making distinctions. There is a difference, the nuances of which I do not have the time to go into in this post.
We categorize people in their sexuality also. We call people either heterosexual, homosexual or bisexual. I think this is a grave error and an opportunity for us to reject and denigrate others, and enter into fruitless political arguments.
What I say is we are all human beings. We are either men or women. Some human beings have same-sex attractions at various levels of intensity or persistence. For that reason, I refuse to all people with same-sex attraction homosexuals because that word categorizes them with something that doesn’t exist really. All the men and women I know who have same-sex attractions are men or they are women who have had or continue to have erotic passions for individuals of the same sex and these passions have had differing levels of intensity and longevity.
It is our moral obligation to welcome and love men and women with same-sex attractions, and it is the moral obligation of all men and women to live out fully the Christian moral standards for sexual activity. What is that moral standard? Sexual activity is reserved only to those who are in a sacramental marriage (the definition of sacramental marriage is something to be described in another post). Sexual activity is not to be enjoyed outside of the marital relationship, and this applies to everyone regardless of the object of our sexual attractions. The reason for this very restrictive moral teaching in Christianity is because the sexual union is to be a physical, emotional and spiritual union that makes possible the good to the partners and the procreation and rearing of children. Thus, the sexual union is to be fruitful, permanent and faithful. This is not only God’s plan, it is a plan obvious with only quick glance at the physiology of the human person.
Categorizing people only demeans them, brings us into needless conflict and misunderstanding, and politicizes what must remain issues of love and respect.