The sixth episode of Book of Circus makes it pretty clear in the silhouette shot that he did penetrate Beast.

while explaining to Wordsmith about the solution of the Phantomhive Murder Mystery, in chapter 49. that I ended up showing humans such a humiliating state. It is canon, as quote 'However, the first time in my long life as a demon.

I'm pretty sure that it's canon that he doesn't like being naked around humans, so there's that, too. Also, all over Kuro fandom, Sebastian always has sex fully clothed. His scene with Beast is rather ambiguous, but it's pretty obvious that he at least had penetrative sex with the nun.

Fangirls would like to think that he just fingered them.

But there is something delicious about the implication that Sebastian is so efficient, even during sex, that he doesn't need to disrobe. which I'm sure many of us have done with gusto. It's likely left unclear on purpose to allow the reader to insert his or her own interpretation. However, both hands are accounted for (one gloved, one not - he'd removed one already because it had gotten bloody from the preceding confrontation with Spears), suggesting either he wasn't using them OR there was some creative position-switching occurring between frames.

The inflagranti delicto scene in the manga IS rather ambiguous.

Note how in both scenes Sebastian seems to be fully clothed and not moving very much, and after he comes back to Ciel in the manga he isn't wearing his gloves because 'They got dirty'. Sebastian doesn't really have sex with either of the girls.He just fingers them.

And if they do reproduce, who's to say they don't do it asexually?.

(And if demons in this series do indeed have genders, why would they be the same as or analogue to human genders?) Additionally, if demons have no need to reproduce, they probably don't have sexes either.

While nothing has been confirmed, at the very least it is reasonably safe to assume he's not a man, as gender is (as far as we know) a uniquely human construct.

What we saw was the closest thing a human could see without going crazy.

Sebastian's true form is an Eldritch Abomination which drives people insane just by looking at it.

Sebastian had no problem with appearing in his true form in front of Ciel in the manga.

Whatever he is is Nightmare Fuel, judging from Ash's reaction and Sebastian's insistance that Ciel shield his eyes.

It's not outside the scope of possibilities.

Given his status among fangirls this would be quite the hilarious revelation.

All we know is that he wears a pair of dominatrix boots. Sebastian, in his true form, is female.Either that, or a crossdresser.

Karin kuroshitsuji book of the atlantic