Newly-knighted Anakin is being his endearingly petulant self and having entirely too much fun picking silly arguments with Obi-Wan on the basis of "I don't have to do what you say, you're not my master anymore." So Obi-Wan ties him up and fucks him into submission. Could be established relationship, could be their first time, but Anakin is totally into it, and there's playful teasing and bantering the whole time.