Enslavement is not that radical of an idea
Slavery is actually not all that radical of an idea. Already, both men and women are enslaved till they reach the age of 18. They have to do whatever chores their parents order them to do, and they can be spanked or otherwise disciplined for disobeying. They are subject to compulsory schooling and can be thrown into detention or juvenile hall if they resist. They also are not allowed to vote, even though they pay taxes (sales taxes when they spend their allowance, and income taxes when they turn 16 and start working). The average voter probably supports all this.
Red pillers favor extending the enslavement of women beyond age 18, under the theory that, especially during some of women's most important years (ages 15-30), they are usually not able to make decisions that are as wise as what their fathers could make for them. In countries like India where arranged marriages are the norm, it does seem like there's a higher marital success rate.
Bear in mind, the term "enslavement" tends to be used very broadly by libertarians, to cover everything from conscription to jury duty. Mainstream society, and the Supreme Court, obviously don't consider that kind of mandatory service enslavement, or it would have been struck down as a 13th Amendment violation.
My point in saying that it's not that radical of an idea, is that it's an idea that by now we have plenty of experience applying in the real world and seeing how it works. One disadvantage that libertarianism often has is that it proposes ideas that have never been tried, and therefore people will say, for example, "Show me one place in the world where anarcho-capitalism has worked."
"Ah, the moral relativism dodge."
That wasn't the way I intended it. Part of my point was that slavery of children has been tried before, so it's a practice that we have some empirical evidence about. One of the problems with a lot of libertarian ideas, such as anarcho-capitalism, is that there's little empirical evidence about how it would work, so people will say, "Show me one place on earth where a society without a state has flourished" and use that as a reason why we can't try it. The subjugation of women to men has also been tried extensively throughout history, so we know a lot about the consequences of it. It wouldn't be an untried social experiment; to the contrary, feminism is the relatively new social experiment, and we're just now starting to get some evidence of the results (e.g. falling fertility rates, diminished levels of female happiness, rising suicide rates among menopausal women, etc.)
The other aspect of enslavement of women is that it is very similar to practices the voters already approve (e.g. the enslavement of children). As a pragmatic political realist who is always mindful of what the voters will be receptive to, I have to take that into account.
"Libertarians generally oppose most of the child indentured servitude bullet points you listed above"
I'm not so sure about that. One of the rationales for why I'm being kicked out of the LPVA is that they say I don't respect the fact that children can't give informed consent (e.g. to sex). The idea that children can't give informed consent is one of the main arguments for enslaving them. Apparently, the LPVA does believe children are incompetent to exercise their rights of self-ownership. If they don't own themselves, then someone else owns them (at least temporarily), which is the very definition of slavery.
If we can justify taking away rights from kids on the basis of their being less competent than their parents or other guardians, then why not take away rights from women on the basis of their being less competent than their fathers and husbands, if that can be proven? The theoretical foundation having been laid, and agreed on by Libertarians at least with regard to children, all that is left is to gather the empirical proof.
"This anti-woman philosophy masquerading as a strand of libertarianism would only exacerbate that problem. As a Libertarian activist who wants our candidates to do well and get elected, I don’t want libertarianism associated with a philosophy that alienates half the voters."
I'm antifeminist, not anti-woman. There are female antifeminists as well, such as Ann Coulter and Christy0Misty, who have said that women's right to vote should be taken away. Mises (in Socialism) and Rothbard (in Power and Market) too made suggestions along those lines.
"As a single heterosexual man, I would prefer to date libertarians rather than statists, so I don’t want libertarianism associated with a philosophy that repels women by casting them as mindless slaves."
You seem to be presuming that (1) equality of women to men is a libertarian view, (2) the idea of enslavement of women to men repels all women, and (3) it's preferable to date libertarians rather than statists. Actually, I think it's often best to have a romantic relationship with women who are apolitical. Thus far, I haven't observed libertarian women to have a very good track record of loyalty; most of them seem to end up divorcing at least one husband.