Juliet is most definitely not treating Paris fairly when she encounters him at Friar Laurence's chambers immediately after consenting to marry Paris. Juliet actually has no reason to dislike Paris. On several occasions, Shakespeare actually portrays Paris as a very desirable match, showing us that the only reason why Juliet prefers Romeo over Paris is because she lets her wild, uncontrolled emotions get away with her rather than using rational thought, illustrating Shakespeare's theme of violent emotions vs. rational thought.
We first see Paris being characterized as a noble and handsome man in the third scene of Act I, when Lady Capulet tries to persuade Juliet to think of marrying him. Lady Capulet refers to him as "valiant," meaning brave and heroic, in the line, "The valiant Paris seeks you for his love" (I.iii.78). Not only that, Juliet's nurse describes him as a "man of wax," meaning a wax statue, which is to say that he is so handsome that he looks statuesque. Not only that, we see that Paris truly does love Juliet. He obviously feels very passionately about her, otherwise he wouldn't ask for her hand in marriage at such a young age, only to be turned down by her father, not just once but multiple times. We especially see Paris's love for Juliet when we see him legitimately mourning her death, saying that he will water "with tears distill'd by moans" the flowers he is strewing her grave with (V.iii.12-15). Hence, since he is handsome, loving, kind, generous, and genuinely caring, she really has no valid reasons for rejecting him; therefore, she is being unfair to Paris in her treatment of him.