You leave out social pressures. While it's not as prevalent today, it wasn't long ago when the difference between male and female promiscuity was black and white. A guy who slept around was a stud, a woman was a whore...and not in the sense that she got paid for it. Like I said, that line of thinking is petering out, but its legacy endures and is ingrained in many minds today. That is at least somewhat responsible for the divide in number of sexual partners, wouldn't you say?
I've never really directly asked how many partners a woman has had before I met her. Sometimes it comes out anyway, but I surely don't hold it against her. Why would you? Because it's not "normal"? Maybe most women downplay how many partners they've had because they don't want to look promiscuous. So is it really a surprise that a more sexually liberated woman, say an SP, wouldn't care as much about conforming to societal norms? And therefore look more promiscuous than the average woman?
Yes, I've probably left out a few more reasons why women, normally, don't have as many sexual partners as men. Whether one reason is more central to a particular woman's decisions is neither here nor there. All that matters for my point is that its out of the ordinary for women to choose to have as many partners as men do. And for women, it's really a choice. Many men would love to have more sexual partners than they do, but they can't manage it for variety of reasons. As a practical matter, women can have as many sexual partners as they want.
As to your second point, if you're not finding out something about your partner's sexual history, you're not looking out for yourself. Her history is relevant to so many things:
1. Are there possible health issues?
2. Can she make a relationship last? Why do her relationships end?
3. Are you her "type", or has she chosen "off menu" for some reason?
4. Are any of her ex'es still in her life in some way?
These are all highly relevant to your prospects of staying together in the long term. You don't need to know micro details about the sex she's had with other people, but you do need to know about her past relationships, where they went wrong, and whether your current relationship with her is different in the ways that matter.
I would say that a woman who doesn't care about your past relationships would be foolishly gambling in the very same way.
If you're just seeing someone for a lark, and you're using all of the precautions, I guess this kind of background doesn't matter. If you are in (what you think) is a serious relationship, they do.
Trying to make this a "woman's equality" issue is a total red herring. If a woman that you have a serious relationship with doesn't want to tell you about her past, it's because she knows there's something about it that you won't like. And, chances are, she's right.