The use of the word "mistress" in this context reflects the social and cultural norms of the time in which the play was written. In Elizabethan England, it was common for men to have mistresses, and this was not seen as particularly scandalous. However, it was important for these relationships to be kept secret, as they could damage the reputation of both the man and the woman involved.
In the play, the relationship between Romeo and Rosaline is a source of great pain for Romeo. He is infatuated with Rosaline, but she is not interested in him. This unrequited love leads Romeo to become depressed and suicidal.
The relationship between Paris and Juliet is also a source of conflict. Paris is a wealthy and powerful nobleman who is betrothed to Juliet. However, Juliet does not love Paris, and she is only willing to marry him because she believes that this is what her father wants.
The use of the word "mistress" in Romeo and Juliet highlights the complex and often contradictory nature of love and relationships. It also reflects the social and cultural norms of the time in which the play was written.