When my wife converted, one of the rabbis on the Beit Din asked her whether she had any Jewish ancestry that she knew of. She did -- a family of Jews that were kicked out of Spain in 1492 and somehow ended up in Scotland.
The Rabbi noted that he asked this of all converts and 90% of them were aware of some Jewish ancestry. Which leads me to think that intermarriage does not reduce the Jewish population as much as thought. Although many intermarried Jews leave, it also creates a pool of children and grandchildren down the road who know of their Jewish ancestry and are pulled back into the religion.
For the record, my wife converted after we met, but before we married. However, she had been thinking about converting before we even met. Meeting me was just the final push she needed.
I hear similar stories - people who converted to Judaism because of marriage but apparently were interested in the religion before they married. I also hear of Jews who convert out and then return later on.