Biology being what it is, men tend to have more muscle mass and be stronger than women and there are also difference in joints and bone structure, all of which affect the load bearing ability. So you can't just scale it down based on height or weight, since *on average* if you had a woman and a man of the same size you could expect the man to be stronger.
Obviously hiking with a pack is not all about muscle strength, but also agility, stamina, aerobic and anaerobic ability, balance, technique, bloody-mindedness and so on. And sometimes the lack of (or perceived lack of) upper body strength especially can be a benefit, as it stops you from relying on your arms and rely more on your legs, which have bigger muscle groups.
As been pointed out several times already, both women and men are able to push themselves to carry loads that are perhaps heavier than physiologically healthy, for various reasons: survival, everyday chores, work, recreational activities and so on. But I took the OP's post to be more of a question on what would be a comparable load in terms of effort and enjoyment when taking gender into account. I think this is an interesting question and even if it's almost impossible to answer it might make us better hiking partners if we consider that others might have other constraints to ourselves. If I have to carry 15 kg (or 10 or 20 or 40) what would be "fair" to expect my colleague or companion to carry, if I wanted us to suffer equally?
I thought the paper on female soldiers in a previous post was really interesting, especially the bit about stride frequency and length. As you up the weight of the pack, you shorten your stride and if you then need to keep up with a taller person, you can get in to a situation where you risk injury by overstriding when you can no longer walk any faster.
I get this when out with my DH. Normally I have to take three steps for every two he takes, but can keep up. When carrying really heavy loads, I take shorter steps and end up going comparatively slower and lagging behind, despite keeping up a decent pace.
There are plenty of women doing solo long-distance hiking and they are obviously capable of carrying heavy loads (including tent, stove, food etc) over long distances, but perhaps the difference there is that they can pick their own pace.
So perhaps the best advice is to weigh down the strongest/fittest/fastest member(s) of the group by letting them carrying a bigger share, so that everyone ends up walking at a similar speed. Surely that would make it a more enjoyable experience all around and can be adapted for any group composition.
