I assume you mean do to the ALS his genitals are not functional? We do not know that to be the case, however that would have little bearing on his ability to vigorously participate in many types of sexual rituals.

When you understand that even amateurs are now experimenting with sexual transhumanism it is not difficult to picture the sort of sexual cyborg potential open to someone like Hawking. The chair alone may be outfitted with stimulatory machinery beyond our comprehension.

Google "sexual cyborg" to see what even amateurs do today -

Now imagine what is possible for Them.