Yeah - the Y sperm die quicker from heat exposure - so tight sweaty lycra shorts would encourage that. My neighbour is a cyclist and has 1 of each, however they claim their son was conceived in winter!! Too much heat will damage both Y and X sperm eventually.

DH's cousin has been a pilot for many years and has produced 3 girls - Y sperm don't appear to like the radiation either!! Interestingly, studies have also found that flight attendants have high miscarriage rates compared with the general population.
I would love a girl next but I certainly won't be exposing myself to radiation unecessarily prior to conception!