Thanks for the question, Anon.
As I discussed before, the crowning of a Queen of Love and Beauty is a very circumstance-specific scenario. As such, it would depend on the knight, his lord, and his lady over whether such a choice would be seen as scandalous.
On the one hand, I could see it being perfectly acceptable in some scenarios. Again, think about the first two people Barristan considers when he remembers how he wanted to give the crown of winter roses to Ashara – the queen “who was not present” and “Elia of Dorne”. Those two were (obviously) married, but their marriages made them Queen and Crown Princess, respectively, of Westeros, the two highest-ranking women in the country. Because the moment of naming a Queen of Love and Beauty is a singular chance for a knight to have the attention of all assembled nobles, a clever and ambitious man might use that opportunity to praise the beauty of the highest-ranking woman present, especially as there was a good likelihood she would be the host’s wife. Such honor bestowed on the lady might help the knight get in good with the host, and because personal relationships are crucial to favor in feudal Westeros, any chance to have a high (or higher) ranking lord think well of a lower-ranking individual is a chance to take.
Now, in other scenarios, I could see where offering the crown to a married woman would be scandalous. The crown of the Queen of Love and Beauty can carry an implicit romantic/sexual tone; certainly, of the four we know, Naerys and Aemon were rumored (even if untruly) to be lovers, Bonifer Hasty seemed to entertain some crush on then-Princess Rhaella, Jorah Mormont explicitly fell in love with Lynesse Hightower at the tourney he won for her, and Rhaegar and Lyanna need little further commentary. It would do rather poorly for a knight to give the favor to a married lady if gossiping tongues already had reason to pair them together; the act might be seen as yet more proof that they were lovers. Imagine if, say, Harwin Strong gave Princess Rhaenyra the laurel in a tourney, after the birth of one or more of her “Velaryon” sons; pro-green courtiers would not fail to see it – and advertise it amongst themselves – as a scandal, an affirmation that Rhaenyra was cuckolding Ser Laenor with the heir to Harrenhal.
The Queen Regent (NFriel)
I think it depends also on how closely the knight and the lady in question are adhering to the forms of courtly love/chivalric romance and their carefully-constructed maxims of **love from afar** that made them safe for consumption.