There are only eleven possible M/F combinations. Six if a man sits out and 5 if a woman does.

Man sits out:
5 men vs 5 woman *
4m/1w vs 1m/4w **
3m/2w vs 2m/3w ***
2m/3w vs 3m/2w ***
1m/4w vs 4m/1w **
5 women vs 5 men *

Woman sits out:
5 men vs 1m/4w ‡
4m/1w vs 2m/3w ‡‡
3m/2w vs 3m/2w
2m/3w vs 4m/1w ‡‡
1m/4w vs 5 men ‡

* Same team split, just different colors.
** Same team split, just different colors.
*** Same team split, just different colors.
‡ Same team split, just different colors.
‡‡ Same team split, just different colors.

There are two possible ways to get an even 5/5 split (in bold) out of a possible 11 different combinations of men vs women. Sure, it gets more complicated if you go into what order they pick or which individual is on which team, but if you're looking purely at the possible combinations of gender splitting, there's an 18.2% chance of getting all men on one team versus all women on the other. Not exactly outrageous odds.

Slice n' Dice's Sigpic Chop Shop 2004