@Dunslax: The setting looks much more plausible if it were located in Europe (probably the UK or France) than in the USA, so technically speaking the FBI would have no jurisdiction at all.
@donm527: Actually, I think Marie's
original reason (i.e. back when they were still at the beastkin school) was much more practical: beastkin are all supposed to be taken in by masters, and statistically speaking the odds of Marie and Mel being adopted by the same master are so low they may as well be zero, as are the odds that the different masters they would be adopted by would be living in the same locality
and would not mind or would even support them having a relationship, so her confessing to Mel about this (which, given how all-loving and nice she has shown herself, would almost certainly lead to her reciprocating even if she only likes Marie platonically) would only lead to them suffering from heartbreak over being separated.
Of course, it's obvious that her reasoning changed when she reunited with Mel and realized through talking with her that she fell in love with her own master.