I agree with both Batman and Cooker,

Endurance swimming relies on endurance (duh!), technique and body bouyancy. Women can excell in each of these areas. Some of the top endurance-Channel type swimmers are women.

But, in sprints, which rely on more muscle and testosterone, men exert their dominance again.

Like other posters have noted, some of the same factors that allow some of the "pure" swimmers to come out of the water first and then lose position to the bikers and runners, are at work against the women -vs- the men.

Just my 2 cents worth.

Tyson