90% of contact/residence decisions are made between the couples themselves, and never reach a court (though in turn 30% of that figure is made up of situations where one parent has no contact at all, and for various reasons never challenges it legally). So blaming the courts for the fact mothers are usually primary carers seems a bit disingenous - it's something couples decide, most of the time, though you can't rule out how much it costs to take legal action as a disincentive, either. Interestingly, though there absolutely are mothers who contact block for all they're worth without reason, more mothers want increased contact than want less, according to a recent study.
The courts favour the status quo, so the parent who took primary responsibility and/or did most of the care prior is likely to carry on doing so after a split. It's not about what adults want or need if it gets that far; it's about what is seen as least disruptive for the kids. Interestingly a recent (2008, Hunt) study on parental satisfaction after the court process found that more fathers than mothers, by a narrow margin, were happy with it. I'd be really interested in seeing the breakdown of how many fathers who apply for residence actually get it, tbh, though without knowing what proportion were main carers before the figure is harder to give weight to, anyway. But the stats do show that 46% of those applying for residence through the courts are fathers, and 44% mothers. That's a higher proportion of women than I expected - be really interesting to know what underlies that figure in terms of circumstances. Fathers tend to apply a lot more for contact than residence.
I do sometimes see threads on MN where a man is at home with the kids and the woman unhappy and she's advised to LTB, by telling him to go. And it always faintly puzzles me that nobody seems aware that he's the one who would probably succeed in forcing her out, if it came to legal action. But there is a large amount of data which demonstrates that couples revert to traditional gender roles after kids arrive; Dad "helps" and "looks after the kids for" Mum. Not always, but really often. I imagine maternity leave sets that pattern up to an extent and it then rolls that way, but it also happens in countries without great maternity provision. It seems cultural expectations are really powerful because it seems to happen even in households that were very equal in domestic labour division before kids arrived.
It's true a lot of men would be horrified by primary care. But it's also true that a lot are devastated by losing that day to day intimacy with their kids. It's a real dilemma, because 50/50 care, as they have automatically in a Nordic country which presently escapes me, obviously has massive advantages... but it can leave the kids feeling desperately unsettled and as though they don't really belong in either home, and that's apparently worse if there are younger siblings of the new relationships who are there all the time. Sadly there's no perfect answer (and often no financial way to create two homes big enough to be suitable for the kids as more than a weekend break, either).
It's a real problem, I think. One without easy answers.