To the extent that a state exists, it should operate with equal treatment under the law of all humans over which it presides. This is a very libertarian concept. Historically, governments have usually sanctioned and enforced discrimination. However, in a libertarian society all humans, regardless of gender, race, religion, sexual orientation, etc., would have equal treatment under the law and therefore equal rights to vote.
Let’s consider voting rights specifically for the moment. The reason why equal voting rights is a libertarian value is because of the concept of “consent” and voluntary interaction. I do not want to send the incorrect message that voting is equivalent to consent and that therefore, as long as we all vote, the government can do anything it wants to because the voting supposedly implied consent. This would be wrong. Even if you vote, the government is still likely to violate your human rights of freedom and property against your consent even if you have a democratic big government. However, voting at least gives a person some small tiny say in how government works. Voting at least gives a person a small ability to approach closer to “consent”. It helps the interaction between government and person to be a little bit closer to “voluntary”, at least closer than if they couldn’t vote at all.