Why is sexual differentiation necessarily costly in every case? There are many sex differences that give an advantage for one gender that would have been an advantage, or no cost, for the other gender, but weren't as strongly required and so don't exist. Sexual differentiation may be less _likely_ for any given trait, but traits tend to pop up on random chromosomes, or the side effects of sex hormones be farther-reaching than necessary and evolution just "didn't bother to correct," and so I'd expect that some traits that don't necessarily need to be dimorphic (but are needed by one gender or the other) might become dimorphic just by chance.
Men have deeper voices than women. What would be the disadvantage for baritone women? You could argue that a deeper voice makes you less easily heard by prey... but then what's the advantage for women to have a higher pitched voice? Why didn't we all continue this trend to speak at the registry of blue whales? I suspect it's one of those things that "just sort of happened" and then it became a positive feedback loop spurred on by sexual attraction as a result of this initial differentiation, and now we're just at a pretty good state spurred on by a combination of random chance, survival advantage, and a little reproductive enforcement.
Increased sexual dimorphism (and a genetic/hormonal configuration that lowers the cost and increases the odds of sexual dimorphism happening for any given feature) is actually a positive thing for some animals depending on their social and reproductive strategies. When sexual dimorphism is high, you tend to see more random differences that pop up between the sexes but that may or may not provide any evolutionary advantage beyond "well, the other sex is just attracted to it now so let's keep this party going." When dimorphism is low, differences generally have to provide a stronger survival advantage.
Evolution isn't always straightforward or "carefully thought out." The hoops it jumps through aren't always costly and don't always have a rhyme or reason. It's often chaotic and and takes weird turns and it's not like humans are at a "final state" anyway. We're evolving.
I'm not _necessarily_ disagreeing with you (although it's probably ultimately easier to look at the physics, not biology, to prove or disprove your point) but I'm just saying that I think the situation is vastly more complicated than "sexual differences are costly." I don't think they're necessarily costly, simply less likely. A subtle, but important difference.
Throwaway for obvious reasons... I worked in a small tech company headed by a conservative religious CEO (may or may not be relevant, who's to say?), which was fine, he didn't really talk much about religion, and we just learned never to mention politics. I never had a bad review, made PRs every day, kept my head down, had been working there for three years.
When I got engaged, I didn't really talk about it -- they had stopped replacing engineers who left the company and I knew they were going through a rough patch, financially, and it seemed like it would be a bad thing to bring up. The cat was out of the bag when I requested vacation for a honeymoon. The next calendar month, I was let go (they did it so that I would get insurance through the rest of the month). The HR person (the only other woman in the company) actually said "Now you have more time to plan your wedding" and "You still have your husband's salary"
Maybe she didn't realize what she said was questionable, maybe she figured they were giving me a good severance package so it didn't matter (not like I could turn around and sue them), but it was still upsetting.
My husband is a social worker who only makes $40k/year, and we live in a city with a high cost of living. So, not a great consolation. Fortunately, in addition to severance, my direct manager was apologetic and gave me a great review, and I was able to find a new job with a 40% raise within two weeks -- even while "planning my wedding"! (Also, the old company's doing fine now, judging from LinkedIn)
But yes, women often do get winnowed out, even today. At my company, we had to decide who to let go in a remote office and one of my co-workers made the comment "so and so has a family to support." I talked to him about it later and he acknowledged that I made a good point, and he hadn't thought about it like that.
But that's the thing -- when managers are trying to "do the right thing" they can often be reluctant to let go of men who they perceive as breadwinners, and women are sometimes seen as making "extra income" for their families. They may view married women as needing the job less than a single woman or a married man. It may or may not even a conscious thing, and it may happen with less frequency today, but it's still occasionally a problem.
On another note: It's also a problem (IMO, a larger one) that women tend to drop out of the workforce on their own accord during the ages of 20-35, when they have children. My husband and I are trying for a family so I'm currently facing this issue, and my husband plans to quit his job and stay home with the kids while I go back after maternity leave.
I admit, it would be a lot easier if we could just go with the social flow and the genders were reversed here. People would automatically know that I was the male breadwinner, I'd have a wife at home like most other men with families in the company, I wouldn't have to take three months of for biological reasons. But I LOVE my career, and it is what it is. We'll figure it out, but I understand why a lot of women don't want to, or may be dissuaded from putting themselves in that situation early in their careers/relationships altogether.
Men have deeper voices than women. What would be the disadvantage for baritone women? You could argue that a deeper voice makes you less easily heard by prey... but then what's the advantage for women to have a higher pitched voice? Why didn't we all continue this trend to speak at the registry of blue whales? I suspect it's one of those things that "just sort of happened" and then it became a positive feedback loop spurred on by sexual attraction as a result of this initial differentiation, and now we're just at a pretty good state spurred on by a combination of random chance, survival advantage, and a little reproductive enforcement.
Increased sexual dimorphism (and a genetic/hormonal configuration that lowers the cost and increases the odds of sexual dimorphism happening for any given feature) is actually a positive thing for some animals depending on their social and reproductive strategies. When sexual dimorphism is high, you tend to see more random differences that pop up between the sexes but that may or may not provide any evolutionary advantage beyond "well, the other sex is just attracted to it now so let's keep this party going." When dimorphism is low, differences generally have to provide a stronger survival advantage.
Evolution isn't always straightforward or "carefully thought out." The hoops it jumps through aren't always costly and don't always have a rhyme or reason. It's often chaotic and and takes weird turns and it's not like humans are at a "final state" anyway. We're evolving.
I'm not _necessarily_ disagreeing with you (although it's probably ultimately easier to look at the physics, not biology, to prove or disprove your point) but I'm just saying that I think the situation is vastly more complicated than "sexual differences are costly." I don't think they're necessarily costly, simply less likely. A subtle, but important difference.