I'm a student, so I haven't worked in the industry yet, but in general, women earn less than men. This still holds true when comparing similar education/experience . It's perfectly okay to complain that women aren't equally paid/treated/respected -- in many circumstances, they're not.
I am curious to see what will happen with the class of students I'm currently with. Our body is about 2/3 male, 1/3 female, though the average ranking and output of the women is, from what I've seen, higher than the men.
From what I've seen as a student, I will give my own input on the reasons you suggested, however:
-Lack of mechanical engineering experience or understanding
This could be possible, but the same exists for the men as well. Along with that, at least 1/3 of the women in this class started in engineering majors, and switched (due to unhappiness/disinterest in the material for some, and inability to grasp subject matter for others). Many men in the class came from engineering backgrounds as well, and many switched for the same varied reasons.
-Lack of interest in how things work, how things are manufactured
Possibly. From what I've seen (and again, this is all anecdotal, and I am in an educational setting, not in the "real world"), a higher percentage of the women come from artistic backgrounds than do the men. I am fairly early on in curriculum, as well, so we haven't covered as much material in terms of manufacturing.
Then again, I don't doubt that if this is the case with women, it will be the case with the men as well.
-lack of historical support of female role in ID?
Ah, now this is one I think is highly likely.
While we do have some women we see in history, we don't have many. When it comes to highly recognizable figures, they're all Pauls and Raymonds and Charleses and Henrys. There are better known contemporary designers who are women, but many of them aren't mentioned in lower level courses. To be honest, all of the ones I know about are from my own research (then again, all of these students should be doing research on their own).
-Physical strength not meeting up to men? (handling tools and machinery)
This one I especially doubt. From seeing work done in our shops, all of the women are very excited to use tools and machinery. Sure, it is daunting at first (and I admit, it took me time to get over my fear of the bandsaw), but it is also rewarding, and from my conversations with other women in my class, we all enjoy working with tools and machinery in the shop.
This is possible, but this is something that needs to be corrected (at least in the US) on a higher level. We do not have guaranteed maternity leave, and honestly, it is unfair for us.
-Doesn't take design seriously enough? ( poor quality of portfolio?)
Like I mentioned in the beginning, the women have a higher quality of output on average when compared to the men in my class. I see more men in my class with cavalier attitudes towards design, and loads of "well i know a guy who knows a guy who will get me a job after school" from them, as well. However, that attitude is not limited to gender -- I heave heard women say similar things as well.
But again, I am a student, and I am interested to see how the women in my class turn out. I have high hopes for them.
My prior work experience is not in a design field, but I have, overall, seen women (and myself) treated less respectfully and with more carelessness than the men were. Men around me, even those with much less experience, had their career and training goals treated with seriousness, while the requests for training from other women and myself were met with excuses and symbolic pats on the head (which is why I left my work to go back to school).
Young women who haven't worked before (except for part-time or summer jobs) may feel quite discouraged if they get into a position and then experience discrimination like that. And, it is discouraging and it's unfair. Is it enough to give up and leave? For some, maybe. I would guess that those who get positions working for a manufacturing company rather than a design firm might be more likely to experience that, but again I do not have any experience in that.