Desertion, in the military and in marriage
There are two kinds of compulsory service in the military. The first kind is a no-desertion policy, that says that once you voluntarily sign enlistment paperwork, you're no longer free to leave the military. You can be locked up if you refuse to obey your commanding officer's lawful orders and deploy and fight as instructed. Libertarians are mostly silent about no-desertion policies. Presumably, if they don't believe such a contract should be enforced, they would think that the disloyal soldier is still under obligation to reimburse his employer for breach of contract, or else he would have committed fraud.
The second kind of compulsory military service is conscription. If you are drafted, even though you didn't sign enlistment paperwork voluntarily, you are still expected to show up and obey your commander until you are released from service.
When marital rape was legal, wives basically were under a no-desertion policy. They had an opportunity not to marry if they didn't want to, but once they said "I do," they had to, for the rest of their lives, open their legs whenever their husbands wanted, or he could rape them (or maybe spank her till she submitted).
Red pill libertarianism favors going a step further to actually conscripting women into marriage, if that's the father's wish. Much as a young man turning 18 has to sign up for the Selective Service, a father may opt to marry off his daughter, at whatever age he considers ripe for marriage; and from that point onward, she has to have sex with her husband or be potentially raped or spanked into submission.
One might argue, "Wait, doesn't libertarianism oppose conscription?" Yeah, of men. That's because it isn't necessary to draft young men; the government (or private defense agency, if we're under anarcho-capitalism) can just jack up soldiers' pay as high as it needs to, in order to give men enough of an incentive to join, even during wartime.
Now, one might argue, "Since marriage is a trade of money (often in the form of gifts and support) for a woman's sexual availability, why don't husbands just jack up the pay they're offering women, to get them to marry them and be sexually available? Why is conscription necessary?"
Because young women aren't known for being rational, long-term thinkers. The whole basis of red pill libertarianism is that women (especially young women) need to have their behavior and decisions controlled by men, because they're incompetent to make their own decisions. In the family, there's a division of labor between the sexes in which men are supposed to do the decision-making while women carry out the decisions. Specialization makes them more efficient.