I first heard this rumour early in the '70s. (I have 2 daughters) If there is any truth to it, it must be a relatively small effect rather than an 80% chance. An 80% chance would be so obvious that someone would have been drawn to do a statistical study.
I will say that at the jet bases I was posted to, there did seem to be more daughters than sons born to pilots. On the other hand, at a maritime patrol base there was no apparent bias.
Possible mechanisms for such an effect could include:
1. g loading
2. high altitude radiation
3. termperature stress in cockpits with inadequate environmentals.
Are there any flight surgeons out there who can comment on an anecdotal experience basis?