But not Facebook. That ship has sailed. I can't and won't trust them.
Region, sure, that makes sense for a local business. But targeting gender or age is discrimination.
At least that way, if done right, you won't be simply invisible to half of the internet users.
You can always find an example of a product that is easier to sell to a specific niche group of people. That doesn’t give the advertiser the right to know that fact about everyone. For the specific case of bras - how would you sell bras for women with large breasts if you don’t know who has large breasts? Do you think that slight inconvenience on behalf of the advertiser should give them the right to keep a database of everyone’s cup size?
Breasts tend to sag with age. As natural elasticity declines, a woman may require additional support.
How are you quantifying this?
You don't even need your "but" (so hold on to it), because this is a classic false dichotomy anyway. Targeting does not require spying nor does spying tend to improve targeting. Basic immediate context (what you're doing, not who you are) works better for both advertisers and willing consumers than personally invasive intrusion does.
You should rather ask whether they prefer relevant ads that put them at risk by aggregating everything about their lives into databases that regularly get breached or relevant ads that don't put them at risk. They can get relevant ads either way. They've just been sold a bill of goods on the need for personal invasion to do it.