Because it is repeatedly condemned throughout the Bible. It's one of the Bible's great consistencies.
Many things are condemned throughout the Bible, so logically this does not answer my question.
Many folks seem to hold to a notion that the Bible teaches that marriage is to be between one man and one woman, and that this is all there is to it. But things get murky. In the OT, we read of patriarchs and kings who had harems; these are not condemned in the relevant passages. (E.g., Solomon is condemned not for having many wives, but for taking foreign wives, who introduced their gods into Israel.) In the NT, we see the practice of levarite marriage: if a woman's husband dies, she marries his unmarried brother (if he is elible). This apppears in the familiar passage where the lawyers ask Jesus whose wife will a woman be in heaven, who has married in turn seven brothers.
One thing is very clear, however. In the NT, it is certain that marriage is to be permanent and it is to be between one man and one woman. And it is to be based on mutual love and caring (having a wife is to be no mere matter of property rights, as was usual in ancient times). Other posters on this thread are also quite correct to interpret Romans 1 as a statement of homosexuality being the result of idolatry. (It is not my intent in this thread to defend the spurious notion that the Bible, OT and NT, does not condemn homosexuality.)
As Dan put it:
The Bible is best viewed as one long, long sentence; and there is, in that sentence, nothing but condemnation for homosexual sex. This fact is unclear only to those who wish it to be unclear.