Is ghusl obligatory when the glans of the penis disappears into the wife's mouth without ejaculation, by analogy with its disappearance into the vagina?

Ghusl becomes obligatory as soon as the glans disappears into the vagina, whether both parties are circumcised or not. This is by consensus of the jurists. As for oral sex, it does not, by itself, necessitate ghusl from janabah.