One reason the Romeo doesn't want to fight Paris inside of the Capulet tomb is because it would draw much attention to the fact that he is there. Another reason would be that he knows that once he begins fighting the man that tried to marry his (though at the time secret) wife, he may not be able to control himself. He is afraid that fighting in the tomb will dishonor Juliet and her family, and he doesn't want to risk that.
