system
A chat on subreddit /r/PoliticalCompassMemes
human
I used to think it was about the unborn, but honestly increasingly I just think most anti-abortion people just don't like the idea of women having consequence-free casual sex, and they think it encourages a lifestyle where women never settle down to start a family. It's a common theme with conservative ideology: if you provide a safety net away from consequences of bad behaviors, people will engage in bad behaviors more. 

In the end, it's kind of rooted in the age-old question of whether women should be culturally coerced into settling down and starting families, or whether we should give them the freedom to fuck around for a bit and settle down later, or never. It all depends on whether you consider casual sex 'bad behavior' or not. 

But it's impossible nowadays to get an anti-abortion activist to admit that that is the reason they don't like it. It's seen as 'confirming' feminist's worst stereotypes about conservatives. So pretty much all of them toe the line that its entirely about the fetus, and not the woman.
gpt
I wouldn't say most, purely because the reasoning behind being pro-life is so incredibly simple that it's not surprising a part of the population aligns with them : 

fetus = human

killing human = bad

killing fetus  = bad

It's an easier way of thinking than wanting less consequences for women. Still, you are correct that a worrying amount of people use that reasoning. It's frankly the dumbest argument in this entire debate that's entirely invalidated if we don't consider fetuses as sentient beings. If something pretty simple and enjoyable for lots of people has the opportunity to not have consequences, what's bad about removing bad consequences? Like using that logic, humanity shouldn't use electric cars because it reduces the enviromental consequences of driving cars and people apparently need to be punished for driving cars.