This has been an interesting and valuable discussion for me (lurker, also Spartacus). I'm a SAHM which I'm very happy with, but I also find I have to over-justify it to random people/friends. Which is problematic. I'm aware of the judging that goes on because it's quite unusual where I am, in Australia, where there are massive childcare subsidies.
There are a number of facets to this issue for me, which aren't relevant for all women.
For me, as an older mother with TTC issues, I wanted to be with my babies. I waited a long time for them. The younger one is only one (older is four), so still wants me with him. I didn't want to put them away somewhere so I could go to work. I know that is an unfair and unpleasant analogy but it's how I feel for myself. I completely understand that other women don't feel like that. Partly it is due to having no maternal bond with my own cold and emotionless mother (which caused PND). It was important to me to be able to be close to my babies all the time.
I do have a career. I am very highly educated and ended up in teaching. This has been fortunate for my choice to be a SAHM. I probably wouldn't have made the choice if I hadn't had such good conditions (and may have been unhappy being forced into using childcare). I get seven years family leave, and back to my previous level at the end. You can use it in one go, or use it in between children (i.e. you can go back between having children, as long as you don't use more than seven years in total). This is because of a historically strong union (which is probably in the process of weakening this condition due to government pressure) with a woman-dominated membership. Women's careers, the policy tries to aim, aren't disadvantaged if a woman chooses to take time out. Of course in reality they are, as most school leaders are men, inverse to the actual teaching population.
Another factor is that it's bloody tiring having babies and young children. If you have a bad sleeper your mind is all over the place. Should I really go to work where I am usually very sharp and precise, and fuddle and fumble around (even though everyone is so understanding...you have a baby, blah blah). It can make women look disengaged. I don't want that for myself. Having also read about the effects of menopause on behaviour here on Mumsnet our biology is really stacked against us. I can see possible menopause in a couple of the more irrational behaviours of older women at work, unfortunately. One has recently had a hysterectomy (something I shouldn't really have been told by a colleague) so I suspect I may be right there.
My partner is one of those men who probably thinks he's a feminist, or believes wholeheartedly in the patriarchy being damaging, but serenely goes about his life without being aware of what it's actually like being a woman. He doesn't 'notice' housework, doesn't forward plan housekeeping, is terrible at managing bills, thinking ahead about dinner etc. I knew that it would fall on me to do the bulk of housework when (if) I went back to work or chose to go back to work. I didn't want to constantly nag or enter into something that knowingly would cause me stress. This SAHM choice means I do all of the house management, although if I ask him to bring in or fold washing, or empty dishwasher, he does it immediately, of course. It's more that me being at home means I'm the manager. He also has a chronic condition (called a disability by some with it) which is managed by medication but triggered by stress. Our aim, or my aim, because it's my choice, is to have as low stress a family life as we can in these early years when screaming babies can make your blood pressure rise. Also I had an unhappy childhood with angry, disengaged, cold parents. I want to do things differently.
This may make me sound like a doormat, because I am quintessentially a 1950s housewife, but I'm quite happy with it. I do feel like I'm in control. I think having strong union conditions (the 7 year family leave) is absolutely crucial, and a lot of the women on here who talk about careers are in the private sector with insecure employment. I can't lose my job if I choose to do this. It's protected.
Baby is fractious now, will continue musing in a little while...