I don't think it is necessarily rude to ask but I'm not really sure why people need to know.
If I toured Abraham Lincoln's home and then asked the tour guide if AL was a heterosexual, I would expect him to look at me funny and change the subject. I see no difference between the two, even though I know people equate g*y with scandalous at times. Just like when you read a bio about someone, if the person is g*y, their bio will state that Super Fabulous Celebrity "is openly g*y", yet I never see it written that a celebrity is "openly heterosexual".
I would say that in some cases knowing if a king or queen was g*y would be relevant because presumed same-sex relationships
can and have directly effected the world around them. Taking the previous examples stated:
Richard I acknowledged at least one son - Haven't read any books on Richard specifically, however as there were no children of his marriage it led to a small war between King John and Arthur of Brittney who was considered to Richard's heir based on the rights of succession; Arthur's subsequent murder is attributed to have been done personally by King John but has never been proven.
Edward II and his queen had four children - his supposed affair with Piers Gaveston directly lead to a war with the barons and the murder of Piers as well as several peers of the realm. As for the supposed affair with Hugh Dispenser the Younger it directly lead to Edward's queen leaving England and then returning, overthrowing Edward, and putting their son, Edward III, on the throne. It's interesting to note that Hugh was castrated in addition to the normal traitor's execution (hanged then drawn & quartered); castration is not a normal part of a traitor's execution and was not included in any of the other executions related to the overthrowing of Edward II including Hugh's own father (who was considered equally as accountable for the tyranny of Edward).
James I and his wife had at least five children - I have a few good biographies but haven't read them yet
So yes it's a valid question as even though nothing can been proven (i.e. "Look we have photographs!" hard to do before the 1860's) because wars have been brought about by subjects thinking a king (or queen) was g*y and that said relationship
was detrimental to the kingdom. I don't think it's right but then we have to remember that we are looking through the viewpoint of living in the 21st century; different ages had different morals and values.
I would think a good tour guide would be able to address the issues, not in depth of course, but at least to understand that it is something that people are curious about and it will come up.
And Waltraud? Can I come visit you? Please?!?!?!?!?