Consider both verbal and nonverbal components of assertive communication.
Li could sit her down and lay the subject out nice and slow to show that he is not pressuring her and then ask "I think it's time to go to the next level in our relationship" without hesitation but not being forceful about it either. He could use good eye contact to make it look like he's really serious about it and positive body language.
Explain how Li and Kate’s well-being will be enhanced by Li’s use of assertive communication, and how positive sexuality is promoted.
Both of their well-beings will be enhanced