I'm speaking of the reasons for their knighthood, not their gender or sexuality, though that's interesting too!
Do other monarchies do this sort of thing?
When did being a knight go from (more or less) genuine servitude to recognition?
Does the Queen herself decide these things, or is there a council?
I answered this with a very brief overview here
I didn't include sources at the time because I thought it fitted in to common knowledge, but if you want to read more there's plenty about the honours system and their history on the British Monarchy websites and Parliament's website.
The main points are
These honours are not given just for being a celebrity, the person has to contribute to national life, like winning a major international sports thing or doing charity work.
The non-military knighthoods, the knights bachelor, pre-date the military ones, the knights chivalry. So the answer to your question is 'from the beginning of the tradition of the current British honours system' as it's the military orders that are the innovation.
The top ranking orders are reserved for knights chivalry.