I've read that boy sperm swim faster but girl sperm live longer. so if you want a girl, bd a few days before ovulation as by the time you ovulate, the boy sperm would've died off, theoretically increasing your chances of the egg being fertilized by a longer living girl sperm.
the theory goes that the closer to ovulation you bd, the more likely the faster boy sperms get to the egg and fertilize it first.
we bd right on ovulation and got a boy.
I do often wonder though if men produce a greater quantity of either sperm as this would be a variable also? like families that have like 4 or 5 girls...surely that's more than just pot luck? I reckon some guys have more girl sperm and others have more boy sperm.
end of the day I reckon it's just chance though. there's no real way to accurately sway either way!