Why is Paris in Juliets room?

He doesn't refer to Juliet as Venus at all. What he does say is:

"Immoderately she weeps for Tybalt's death,

And therefore have I little talk'd of love;

For Venus smiles not in a house of tears."

He means Venus would not smile ON HIM in a house of tears; in other Words, the goddess of love would not favor his pursuit of Juliet right at the moment.

He means that he knew he could not properly court Juliet, and win her heart as her father had instructed him to do, because she was too grief-stricken (supposedly over Tybalt's death), and wouldn't have appreciated hearing this guy she barely knew talking about love and marriage.