I tend to gravitate towards female characters in single player games and I ask myself this same question a lot. The weird thing is that I have no gender preference when my options are more limited - it's only when I'm being asked to customize my appearance. With male characters I never really know what I'm looking for - usually what looks right is what looks familiar. Maybe I'll be scrolling through hair options, find something that reminds me of Aragorn, and before I know it that's who I'm roleplaying. With a female character it's easier for me to resist these influences and make someone original because I can rely on my own preferences to tell me what looks right. I'll still play male characters if I have a specific influence in mind, but otherwise I find myself much more likely to actually finish a game as a female PC.