These are valid questions and objections; I can certainly understand why people would be confused because it does appear that we are selectively culling the scriptures. I would like to offer an explanation of why conservative evangelical Christians teach that laws against homosexuality should continue to be enforced in today's society.
Within the Jewish religious system the laws laid out in the scriptures can be categorized into several different areas. There are Levitical laws, ceremonial laws, cleansing laws, moral laws, and sacrificial laws. Levitical laws related directly to the conduct and life of the Levites. Ceremonial laws dealt with feast days and hygiene. Moral laws dealt with issues concerning adultery, homosexuality, incest, bestiality, prostitution, giving to the poor, stealing, deceit, perjury, dishonesty, treatment of the disabled, etc. For conservative evangelical Christians like myself, we believe that the moral law from the Old Testament is still applicable today.
We believe the moral laws to be applicable today because the New Testament reinforces the application of the moral laws while at the same time it teaches that the sacrificial laws are no longer needed.
Christ taught that the moral law is still applicable. Consider John 8 and the woman caught in adultery. While Christ did forgive her, he did not excuse the sin. He told her to go and sin no more. He recognized it as sin and in effect was enforcing the moral laws of the Old Testament. Christ also taught against adultery - again the moral law - in the Sermon on the Mount in Matthew 5:27.
(To be continued...)