It is the nature of women to have a child inside of them.
Perhaps you can understand why so many women find this particular assertion offensive.
I don't know how we are defining "so many" here. I can see how some women may be offended by the proposition that only people that have babies are women, but I think what is being proposed is that only women can become pregnant.
I know that this is debated in some circles, and that the counter argument is that the argument is circular, but that is the nature of a definition. But I don't know that "so many women" would find it offensive anymore than men being offended by the proposition that they cannot be pregnant.
Personally I am pretty flexible on abortion. It is probably an issue that I have kind of gone back and forth on through the years. I can see both sides and would probably call myself both pro-life and pro-choice.
I personally believe that regardless of how you define a fetus or a zygote, if it is a "person" or isn't, which would require concrete definition of "personhood" to begin with, I can most definitely assert that a zygote and a fetus are future persons. Whatever a person is, a zygote and fetus will one day become a person, unless something like a miscarriage or birth defect or other complication occurs. Every "person" you have ever met and talked to and had a conversation with was once a fetus and a zygote. I was. You were. As a strict Materialist, Tom, I would think you would appreciate the idea that a "person" was nothing but a group of cells, rather than any mystical connotations of the soul, etc. There is a direct line between the cell of a zygote and every cell in every "person"'s body. When you destroy a zygote or a fetus, you destroy a future person.
I am of the opinion that most morality is generally four dimensional. That is to say, the reason something is immoral is often not because of the immediate action, but because of its results. Sometimes the law is less clear on this distinction, but sometimes it is. If I killed a 2 month old baby, I've taken away it's future. Were I to poison you by flooding your home with carbon monoxide or other toxic gas, you may slip gently from unconsciousness to non-existance, never knowing the difference. But you won't be able to wake up again and have another day.
Because of this I'm generally anti-abortion. Certainly I don't have a problem with the procedure in the case where the mother's life is in question. At that point it should be up to her. But I don't know when we start talking about mental health of a mother. You can stretch that pretty far.
The flip side is that I recognize the concept of bodily autonomy in the law, and I recognize it's value. I recognize the general desire to not have the government be able to tell people what they can do with or to their body or the government take control of someone's body. If a newborn baby immediately required a blood transfusion and the only person that could provide that transfusion would be the mother or father, I recognize that the law cannot force either the mother or father to give blood. Or donate an organ. I recognize that the government cannot force organ donation or blood donation regardless of the recipient is a child of the prospective donor or not. I recognize the general immorality of the action while recognizing the view that granting the government power to force these things is dangerous.
What is funny to me though is that Democrats as a whole are really not that libertarian anymore. They may have been in the 60s and 70s when the sexual revolution occured. But not so much anymore. The Democrats and liberals in general are more about preventing harm and preservation of life these days. Regardless of how it impacts personal freedom. So if a starving child were to plop itself down on my front yard, the Democrats and liberals would generally proscribe that it was my duty to feed that child, and tax me accordingly. They would take my food and give it to the child. A conservative or Republican may see the morality of feeding the child (at least they used to, maybe not anymore, depending on if the child was Mexican or not) while saying that it should not be in the governments power to force me to feed the child.
In turn if a baby vampire attached itself to my neck and needed to stay attached for 9 months to survive, a Republican might argue that it is within their rights to shoot the little sucker, or have it forcibly removed, while maybe a liberal or Democrat would say otherwise.
As it has already been pointed out, Republicans are now the party of "don't force me to take a vaccine" while Democrats are the opposite in many cases. The general layout of the values of the parties does not jive with their positions on abortion and bodily freedom. Republicans are more libertarian now and more about bodily freedom than Democrats are in many cases. People like to point out the seeming hypocrisy all the time. Republicans want to take care of fetuses but not feed children. Liberals want to force parents to feed their children, vaccinate their children, but don't want to force women to have a child if they are pregnant.
I think the answer is that when the whole position began, the values were more reversed. The conservatives of the 60s and 70s were in fact more restrictive on personal freedoms than liberals were. In some cases they still are (drug use, homosexuality) but conservatism is rapidly evolving and so is liberalism. Since then, women in general and feminists in particular have been a part of the Democratic Party coalition. It's a democratic interest group, and I think the Democratic party was basically built around a coalition of interest groups in the 70s and 80s, while the Republicans in the 70s and 80s remained more broad in aims.
Now, I'd like to close with some observations that have been made that I think are valid. The first is that motherhood, or pregnancy, is a particular condition. Yes, only women can get pregnant. Only women can have periods. Only a woman can fake it. Only a woman can look sexy while drinking a cosmopolitan.
This may seem unfair, or unjust, but it just simply is. This is the maddening aspect for some liberals because I understand that sometimes this is used as an excuse for injustice. "That's Just The Way It Is". But in this case, I do not believe it is a matter of justice. It is simply biological destiny. You may not like it, but life and nature in general is not just. This is also infuriating for some liberals, but one of the aspects of conservatism (at least in the past) was to look on the brighter side of life. I don't believe in forcing women to become pregnant, but I also don't think that it is socially unjust that only women can become pregnant.
The special aspect of pregnancy makes it a little different than refusing to take a vaccine or not wanting to feed the poor or a vampire baby attached to your neck. Pregnancy is essential to the survival of the human race. It is in fact a good thing overall. It may not be seen as a good thing to a prospective mother, but it is still a public good. Pregnancy is a miracle. It is, in my opinion, not a curse, but a special privilege and trust given to women. This is of course looking on the brighter side of things rather than focusing on how unfair it might be.
Finally, I would like to point out some of the flaws in the libertarian argument when it pushes into "the government can't" realm. While it may not be good, or should be limited in general by conservative standards, bodily autonomy has limits. The government can indeed force you to do things with your body. The government can draft you and force you to go to war. They can sit right next to you with a gun pointed at your head and tell you to go march off to the sound of the guns. It has happened.
The government can indeed force you to take vaccines. It has in the past and still does in particular cases. Military personnel. Medical personnel. Travellers. Immigrants. School children. Teachers. Etc.
The police in many states can indeed forcibly take your blood if you have been arrested on suspicion of DUI. The police can forcibly take DNA samples if you are charged with a felony.
In all of these cases, the general rule is that when the good of the general public is threatened, your bodily autonomy can be sacrificed. This is not particular to women, only pregnancy is. In fact, until recently only men could be drafted.