Posting this again cause it was removed for some reason.

Actually this is usually down to the fact that men don't fight for custody as often as women do. We can put this down to them adopting traditional gender roles rather than biased judges.

When men do fight for custody, they win 63% of the time

When fathers contest custody, however, studies consistently document that they win at least half of the time. A Los Angeles study showed that when fathers contested custody, they won 63 percent of the time; a Massachusetts study found this to be so in 70 percent of cases. And a 1997 article reviewing custody laws from the 1920s to the 1990s concluded that “when fathers fight for custody they have always had about a 50 percent chance of winning, no matter what arguments or what experts they employ.”

http://povertylaw.pbworks.com/w/page/17976115/perils%20of%20joint%20custody

We began our investigation of child custody aware of a common perception that there is a bias in favor of women in these decisions. Our research contradicted this perception. Although mothers more frequently get primary physical custody of children following divorce, this practice does not reflect bias but rather the agreement of the parties and the fact that, in most families, mothers have been the primary caretakers of children. Fathers who actively seek custody obtain either primary or joint physical custody over 70% of the time. Reports indicate, however, that in some cases perceptions of gender bias may discourage fathers from seeking custody and stereotypes about fathers may sometimes affect case outcomes. In general, our evidence suggests that the courts hold higher standards for mothers than fathers in custody determinations.

Massachusetts Supreme Court’s Gender Bias Study
http://www.amptoons.com/blog/files/Massachusetts_Gender_Bias_Study.htm

These are US based studies but I think our cultures are sufficiently similar.