Larry - actually I think it's more of an issue in classical music, because legato phrases are often longer and written that way into the score, so the player gets less choice about where to breathe.
I also think we need to be careful talking about "strong" muscles. Most of musical excellence on various instruments has much more to do with fine coordination and control of muscles, than with sheer physical grunt. I don't know of any evidence that men have "stronger lip muscles" than women to any significant extent that would affect bess playing, and I don't even know whether brass players have stronger lip muscles than non-brass players, or have just learned to use them more effectively.
Further to Lurcio's correct points above, the french horn is considered hard among brass instruments because of the closeness of the harmonics and skill difficulty in distinguishing them aurally and physically, not because of anything to do with physical strength.
About the only innate difference I can think of that might affect all this is lung capacity. Men do on average have more of it than women, but then it also varies with lots of other physical factors as well. It also doesn't seem to have stopped Alison Balsam or the many excellent female brass players in music colleges. It would be surprising if they were all perfectly capable of getting that far, but then innately unable to play in professional orchestras because they can't sustain long notes well enough.
It would seem likely that the main factor is laddish male brass playing culture. Male chauvinism is gradually dying out in classical music but it hangs on in some areas (like Austria, or brass sections generally) more than others. It can also be exacerbated by the insular "not what you know but who you know" nature of classical music. When 99% of gigs are booked over a pint at the pub after the previous gig, who you go for a pint with and how gender shapes your social life makes a big difference.