Here's a question often asked, and it can be asked about many things. Besides misunderstanding the nature not only of biblical authority but of communication itself, the question is typically geared toward defending a practice that is likely questionable, to begin with. After all, we don't ask this question if we know we are on solid ground for practice. If positive authority can be supplied, we will supply it. The question is then pointless.
Part of the mistake of this type of question is the misunderstanding of how communication works. It is essentially arguing that in order for something to be "wrong," it must be specified as being wrong. How many of us communicate this way? "But you didn't specifically say not to throw these hot coals at my brother's face." Conversely, many argue that in order for something to be authorized, it must be specified. Neither is true, and neither will be consistently applied.
The Scriptures both provide and deny authority through its various principles without spelling everything out in detail, and the Lord expects us to grow mature enough to figure this out.
"But solid food is for the mature, who because of practice have their senses trained to discern good and evil" (Hebrews 5:14).
"Where does the Bible specifically condemn that?" is aimed at wanting to practice something when the questioner knows that positive authority is absent whether through specifics or principles. This is not a mature question.
Sadly, when people are committed to practicing something, a question like this will not be enough anyway. If they want to practice something badly enough, they will find a way to practice it (as has been well seen in the same-sex practice debate, though Scripture clearly condemns it). They may take solace in the idea that the Bible doesn't specifically condemn what they want to do, but neither can they provide biblical warrant for it. That doesn't matter. They want to do something, so they'll do it.
In the end, we still must ask, are we seeking God's will or our own? Are we seeking to glorify God or ourselves? Are we speaking from God's authority or our own?
"He who speaks from himself seeks his own glory" (John 7:18).
If that doesn't give us enough pause to be careful, what will?