The question should be reopened
I'll leave to one side the point about moderators, which is discussed in the comments.
Should the question have been closed as primarily opinion-based?
Ultimately, no. I can understand why the question was closed originally, as in its original form it was unclear. I can also sympathise with those who wanted to close it as opinion-based, since the events it describes don't occur in canon, nor are they hinted at in any way. Nevertheless, the question once edited was clear and objective: did Sirius procreate whilst an Animagi? The question was reopened, and rightly so.
Should the question have been closed as being off-topic?
Absolutely not. The question currently has the following close notice:
This question does not appear to be about science fiction or fantasy within the scope defined in the help center.
This isn't true, and we all know it. The question clearly is about sci-fi and fantasy, given that it's about Sirius Black and his activities as an Animagi. The question was not closed because anybody thinks that it is not a question about sci-fi and fantasy. Let's at least be honest about that point.
I suspect that the real reason the question was closed was because its tone and content were determined to be distasteful, overly graphic or obscene. This is reflected in Valorum's comment, which garnered six upvotes.
I'm voting to close this question as off-topic because questions about bestiality are not suitable for a family-orientated website.
Yet this isn't the close reason that was given (not sci-fi or fantasy). Closing questions under by artificially pretending that they're something that they're not is not helpful or constructive. The priority for some appears to have been to close the question by any means necessary, regardless of whether the close option selected was applicable or not. The question is about sci-fi and fantasy and so should be reopened, regardless of whether it's flavour of the month or not.
Of course, finding something in poor taste is a perfectly valid reason to downvote a question. It is not a reason to close a question which is otherwise on-topic. By the way, I'm not seeking to defend the question, which is bad and deserves the downvotes it's getting.
What should happen next?
As I see it, two things need to happen.
Firstly, the question should be reopened. The close reason is inappropriate and should be reversed.
However, the community has clearly made its view clear that these sorts of questions are not welcome on the site. The problem seems not to be with questions about mature themes, sexuality or bestiality, all of which have precedent on the site. The problem is with sexual questions which are unnecessarily gratuitous and salacious - and which are nowhere suggested in the source material. Since this isn't the first time this topic has come up it would seem to be an ongoing problem. The community wants a way to close these questions but there isn't one in the current set-up. I'd suggest a new close reason, probably to be decided upon in a new meta question, along the lines of 'This question concerns sexual acts which don't appear in the source material'. We should edit the description of on-topic topics to specifically debar these questions. This would allow all users (including new users) to have a means of knowing that these questions aren't welcome and the users who don't want them a means of validly closing them.
Should the question be deleted?
Since it has two delete votes I thought I'd add my two cents on this as well. No, it shouldn't - partly because it would involve losing ibid's strong answer.