This opposition comes not out of condemnation, but out of love and concern for the persons involved. Homosexual activity is dangerous to those who practice it, on several levels. It is obviously dangerous physically. AIDS has killed hundreds of thousands of homosexual men in the past 15 years. The condom is even less effective in homosexual activity than it is in heterosexual activity.
Homosexual activity can also lead to other diseases.
Homosexual activity is also dangerous spiritually. God designed sex to be used in the context of marriage, for a man and a woman to give themselves to each other and bring a family into the world. To use the sexual function any other way is a sin. All sin, including homosexual activity, cuts a person off from God. It is choosing our own way over His way.