The Jewish people, more than any nation in the world, abide by the strictest laws in the area of modesty. The net result is that there is much greater joy in a Jewish marriage as well as longevity, inasmuch as the marriage is concerned.
When our Rabbis tell us that there were no days, in the past, like Yom Kippur and the 15th of Ab, one reason is because on those days the sins of the Jewish people were forgiven. That is why the 15th of Ab was considered to be a particularly auspicious time for weddings.
Since the happiness in a Jewish marriage is greater, on account of the laws of holiness and modesty which accompany it, our Rabbis tell us that the groom is forgiven all his transgressions on his wedding day. In this way, the couple enter into marriage with a clean slate, without the shadow of the past following them.
(See Sefer HaToda'ah)
Tu B'Av, Tu BeAb, the 15th of Av, 15th of Ab, weddings, modesty