Anecdotal observation -- just for fun!
Okay, so really nothing new here, but just support for several other theories atomic already has.
I live in a community that has a lot of conservative religious folk (no premarital sex) and it's common to take birth control for a year after marriage but also common to ttc immediately.
So what I noticed is that the couples who get pregnant very soon after being together seem to have their first child be a boy. So of the couples I can think of (5) who did not have premarital sex and got pregnant within four months, all of them had first born sons. I know three couples who got pregnant in 5-6 cycles and two had first born daughters and one a first born son, so take with a grain of salt because we're talking a difference of one month here.
My little anecdotal observation: the longer that a couple has been having sex (with each other) whether ttc or using contraceptive devices, the more likely they are to have a girl.
Exception: one night stand babies tend to be girls (3 out of 3 that I know, 2 were from the same woman, different dads).