Cleveland Kent Evans, who has written numerous name-related articles* claims that any name ending in the "-ley" sound is doomed to become feminine in the United States. While it is true that such names (or any name ending in the long "e" sound for that matter) which are traditionally masculine are more likely to become unisex, Evans is ignoring an important trend: The rising generation of new adults (the Millennials) are more likely than recent previous generations to not let a name's usage on girls deter them from giving it to their sons (which I've posted about before). *I previously mentioned Evans being president of the American Name Society, but he informed me that he no longer holds the position so I edited it out.
What got me to post this is that Evans has been predicting for several years (by posts he's made on sites such as http://www.babynames.com/ and http://www.behindthename.com/) that the name Riley will follow a similar path to other past surnames that end in -ley that became popular as first names (such as Ashley and Shirley) and become almost exclusively female in the U.S. However, I think that Riley is much more likely to stay unisex (albeit more popular for girls) than those other names because Riley peaked for boys at a much higher rank and hasn't experienced a fairly sudden drop compared the other examples (probably in part to my theory based on the generations). Evans's prediction may have had some muster 10 years or so ago when Riley was beginning to level off for boys and increase for girls, but since the name hasn't fallen much for boys it appears that his prediction is only semi-right (the name did become more popular for girls, but not to the extent he thought it would).
In addition, if Jennifer Moss from babynames.com is right, "crossover" names for girls in general are starting to level off in use (probably from the same Millennials that want to keep names on their original gender). Therefore, I think that unlike what Evans predicted I think there will still be plenty of male Rileys being born (although plenty of girls will be given that name too). Note that in my blog post from last July that I linked to earlier in this post I mentioned that Moss originally tended to be against unisex names for boys, but now her opinion appears to be changing some with the times (at least by predicting that the trend for girls will be falling).