I want to post a question about the plot of Iolanthe, but noticed there were no tags about the duo.
Is this the right Stack to ask this question on?
Let’s see how these apply to Iolanthe:
Fairies feature prominently in the plot:
Twenty-five years before the beginning of the opera, the fairy Iolanthe committed the capital crime (under fairy law) of marrying a mortal human. The Queen of the fairies commuted Iolanthe’s sentence of death to banishment for life on the condition that Iolanthe left her husband and never communicated with him again. After the passage of 25 years, the fairies, still missing Iolanthe deeply, plead with their Queen to pardon Iolanthe and to restore her place in fairyland (“Tripping hither, tripping thither”).
There are spells:
Offended, the Fairy Queen pronounces a magical “sentence” upon the peers: Strephon shall not only become a Member of Parliament, but will have the power to pass any bill he proposes (“With Strephon for your foe, no doubt”).
So Iolanthe is on-topic.
However, this would not be true of all Gilbert and Sullivan operas. Most of them, such as Yeomen of the Guard and The Pirates of Penzance, are simply not SF.
I've never seen Iolanthe, but Wikipedia says (emphasis mine):
In the opera, the fairy Iolanthe has been banished from fairyland because she married a mortal; this is forbidden by fairy law. Her son, Strephon, is an Arcadian shepherd who wants to marry Phyllis, a Ward of Chancery. All the members of the House of Peers also want to marry Phyllis. When Phyllis sees Strephon hugging a young woman (not knowing that it is his mother – immortal fairies all appear young), she assumes the worst and sets off a climactic confrontation between the peers and the fairies.
Seems perfectly on-topic to me.
I don't think I'd go so far as to say that Gilbert and Sullivan are on-topic in general, but at least this play looks like perfectly fair game.