Book with gender-disguised male magician, forbidden language, one character can talk to horses

Looking for a book we read back in the early 2000s (2003?) which had a male magician who disguised himself as a woman and was travelling on a quest with two other women, who were unaware of his real gender. Other remembered details follow:

• probably written in the 1990s
• the male mage (while disguised a woman) was involved in a sexual encounter/orgy with a stable boy, and his secret identity was, er, noticed
• high fantasy, felt inspired by Lord of the Rings
• there was a language which at least one character knew, which was forbidden to speak out loud (similar to the reluctance of people in LoTR to speak out loud the language of Mordor)
• one character might have been able to talk to horses
• the cover of the book was purple
• don’t remember if the author was male or female.
• possibly a young adult book

Thanks in advance for any hints and help.