...and in mere seconds, Momonga realized the sad disadvantages of being a skeleton. The only boner that he can get is in the mirror.

Drewko said:

...and in mere seconds, Momonga realized the sad disadvantages of being a skeleton. The only boner that he can get is in the mirror.

Which is bad luck, really. Even our closest primate relatives still have a baculum, but us? Nope, sorry guy.

Drewko said:

...and in mere seconds, Momonga realized the sad disadvantages of being a skeleton. The only boner that he can get is in the mirror.

In book 1 of the light novels (before even stepping out of Nazarick for the first time), he inspects himself in the mirror, and says, "Lost it before I ever got to use it, hunh?"

If you look through Ainz fanarts, you'll see more than a few jokes referring to the fact that he's completely incapable of having sex with any of the attractive women who want to do it with him.