The reason we do not stone rebellious teenagers to death is not because we do not think it is wrong to be so, but only because, like ceremonial laws, a change in covenant does not necessitate or allow that the church keep the civil capital penalties. Thus the church itself opposes homosexuality but does not kill them, but seeks their salvation, for there is room at the cross for all who who turn to Christ for salvation and a new life following Him. But the church may support the state to justly penalize immoral behavior, though the reality is that Christian are the ones who are in danger of penalized for non-violent opposition of the current course the West is on.
As for why it is wrong, one verse would suffice for us who find the Bible to the supreme moral authority, and history testifies that obeying His laws are to our benefit and to our collective hurt when forsaken. See [here] for some current stats on the temporal cost alone. and Homosexuality and health
God created man and women uniquely compatible and complimentary, and they alone are joined by God in marriage, with opposite genders being specified by both Genesis and personally by Jesus Christ. (Gn. 2:18-24; Mt. 19:4)
The Bible only condemns homosexual relations - by design and decree, in principle and by precept - and never sanctions them wherever they are manifestly dealt with, and the injunctions against them are part of the transcendent and immutable moral law. (Lv. 18:22; Rm. 1:26,27)
As regard Homosexuality and biblical interpretation, pro-homosexual polemics, in all their prolixity, are spurious, and they ultimately require the use of a hermeneutic which would negate any moral law, or negating the authority of the Bible. An extensive examination and refutation of such attempts can be seen here: HOMOSEXUAL RELATIONS and the BIBLEDaniel1212 15:48, 26 July 2011 (EDT)