Powerful magic can be offset by requiring the caster to bring their own energy, i.e. exhaustion that might lead to death. Or it might be offset by the weight of responsibility, because as you know great power begets great responsibility. But that's not quite your question. Your question is why would women willingly turn down such great power? And I'd like to challenge that:
Why should there be a price to pay?
Your problem, as I understand it, is that half of humanity suddenly becoming super powerful is a very different setting from what you're trying to achieve, where magic is much more confidential. And the cause of your problem is that your mechanic for achieving magic is self-determination. That's a dangerous mix.
Typically, in fantasy becoming a wizard is usually a function of how you're born. It's genetics or it's divine, or it's generally something that bears no explanation. Some people are magic, some aren't, and you can set the rate arbitrarily and don't have to justify it. The reader just accepts that is it what it is.
That points to two potential solutions:
Pivot your mechanic.
Maybe it's conventional, but "you're magic because you happen to be born that way" works.
You could tweak that by adding an element of having to willingly listen to your inner magic in order to unlock your powers. Simply put, some people are magic, and some aren't, or they don't know they are, or they don't fundamentally believe they are.
Here's one example: Luke Skywalker having to willingly turn off the targeting computer and take a leap of faith. He is a great pilot because he's strong with the Force, but he can't materialise that power until he starts believing in it.
Here's another example: Neo having to willingly believe that there is more to the world than what he can perceive, and swallow that pill. The Thomas Anderson of the begining of the movie wouldn't be able to stop bullets because he doesn't believe that's possible.
It's taking that leap of faith that makes them powerful. It's not very original, but you know what they say about reinventing the wheel: don't.
Pivot your setting.
Maybe most of the women of your world are just magic and quite powerfully so, and it's going to be a radically different world. That opens a lot of great themes, and also incredibly complex themes, of gender roles, gender power dynamics, and so forth. Easy stuff.
But why though?
This is where I'll develop why it might be very undesirable to set such a price. And I want to be clear: this isn't an indictment on you, or anybody in particular.
I believe that witch mythology exists on a very slippery slope. Putting together power, beauty and fertility has the potential to devolve very easily into something incredibly sexist. Take a traditional witch representation: powerful, independent, knowledgeable, ugly, older, uses her sexuality, unmarried, hates children, some shade of evil. Now let's look at the reverse: good, motherly, married, chaste, young, beautiful, uneducated, dependent, weak. A society that produces that myth tells us what its vision of the ideal woman is.
What I'm trying to communicate here is that how you characterise your witches, how you frame them in your society, and how you frame that society in your story, it is going to reflect on you, the author. It is <current_year> after all, your story/setting doesn't exist in a vacuum.
With all that said, what you are looking to establish here is what is more important than power for womankind. And we're talking about the power to pursue their own aspirations, or to emanticipate themselves, or to burn society to the ground and rebuild it in their image. If you oppose that to "leading a normal life in this pre-industrial fantasy society", motherhood, or beauty, that's the slippery slope I mentioned earlier.
And to be perfectly honest, unless your pre-industrial society is a perfect equalitarian paradise, I can't think of a good reason to not just pay the price.