[Note: This is a revised version of a post I wrote on a “Christian” dating web site many moons ago, in response to man’s thread regarding all the cleavage and other immodesty in women’s pictures on the site.]
Many Christians have a false and wrong division in their minds: God speaks to spiritual and eternal things, but God does not speak to earthly and temporal things. The truth is that God speaks to all aspects of life. (2 Tim. 3:16)
Therefore, many Christians do not even think to look to the Bible as a source of a standard on how to dress. So, the default standard for Christians is exactly the same as non-Christians: Hollywood, their friends, magazines, TV commercials. The Bible is the Best standard, and for Christians should be the only standard because God speaks to every area of His creation. We should be set apart (Lev. 11:44; Eph. 1:4); Christians supposedly know something the world does not know. Let's show the truth of that, so there is no "supposing" about it!
I also find it troubling that Christian ladies are often blasé about being a stumbling block for their brothers, which is sin on the woman's part (Lev. 19:14; Rom. 14:13; 1 Cor. 8:9; 1 Jn 2:9-10).
To dress "sexy" in public (church or anywhere) is to dress like a harlot (Prov. 7:10) because you are seen sexually by every man who even accidentally lays his eyes on you (Ezek. 16:14-16). We are sexy for our husbands in private (Gen. 2:25) because sex belongs in marriage only (Prov.5:15-20), according to God (Ex. 20:14, 17; Mal. 2:13-17). We are not married to every man we pass by on the street; we are not married to every man in church. We shouldn't cause a Christian brother to lust (2 Sam. 11:2-4); we need to do what we can to protect his heart, his purity (1 Tim. 5:2), and his "eye-gate." (Job 31:1) First of all, modesty is a heart-issue (1 Pet. 3:1-4).
Many Christian ladies have bought into the world's view that her sexuality is for public consumption and there is no other way to "make" a man want to marry her; nothing could be further from the truth (Prov. 31:10). God says marriage first, then sex with our own spouse after marriage (Matt. 1:18-20). When a woman's heart is modest, her outsides are modest as well (Prov. 4:23).
Christian men could help the situation by pursuing modest women to marriage, instead of pursuing immodest women to…what, exactly? Or do I not want to know?
A good book (really more of a long booklet) addressing Biblical modesty and the history of our current culture in America is Christian Modesty and the Public Undressing of America by Jeff Pollard.