@Thelonewolf @mikureimu She could also just not be being very honest. After all she is significantly older than 41 as well.
Along with Kazane, Chronoire, Medusa, Alcina, and Evermillion, she is an ancient witch of immense power and effective immortality. Which is why she had to be sealed away previously. Like Medusa and Evermillion, she cannot be killed easily if at all. In the arc where Honoka and Ayaka revisit their childhood memories, it is mentioned that she was the dreaded nine-tailed witch who had been wanted by the Workshop for centuries. She was merely masquerading as Kagaari Kayou.
On the other hand, she may not even be human, and thus may have considered every other human male unworthy of her and simply abstained from sheer arrogance. It would fit the character.
I'm interested to see what the vessel thing is about. In the old arc with Ayaka, I'd assumed she intended to transfer herself into Ayaka's body Flemeth & Morrigan style. But now I wonder why an already immortal witch would need to do that. So I think instead now, that it has everything to do with Evermillion. We hear that Evermillion has been used by countless evil witches in the past and has corrupted every single one of them. We also know that Evermillion's appearance and behavior in her demonic-possession-form reflect the quality of the soul she is inhabiting. If Kayou took Evermillion into herself she would become a reflection of all the evil in Kayou's soul and would constantly try to consume her soul. Where she is in Honoka's soul she has had multiple opportunities to consume his soul/body but has chosen not to as her nature is now a reflection of Honoka's own. Therefore, if Kayou can strip Honoka of his independent will (what I believe she means by purifying him) she will have a near-infinite magic battery at hand at all times bound to her through magic contract (marriage/consummation) and with no risk of the demon eating either her soul or Honoka's.
@Meepster It's worth the re-read. Getting it in slow drips, it's easy to lose the thread, but going through it all at once, I was delighted by how well it all ties together.