You wrote the flat was advertised as unfurnished, so why couldn't you object to the landlady she has been misleading readers of her text.In France it is called "publicité mensongère" to say the extreme. The owner might want to feel more secure with a standard furnished flat contract because of some particular clauses to her obvious advantage...No need to argue but you may enquire to check the last rental contract and walk-through report with the previous tenant in which you will find out whether things are right or wrong. You ought to raise questions and ask for details before you sign a contract. Once you approve you cannot modify or withdraw.

I believe you better compare the two standard flat contracts, one with furniture and the other without furniture pieces.

Then you decide by yourself whether to sign according to the owner's request (whatever her motivations be) because you personally like the apartment so much  OR rather look for something else.

Simple.

Yes Alfi, in the Geneva and neighboring France housing market, that's basically the owner's view - it's simple - take it or leave it.

I assume the landlord is not trying to charge you extra rent now she wants it furnished, so she is doing it because the conditions for renting are different between furnished and non-furnished.  A furnished lease in France is usually one year and the tennant only has to give a months notice, and the landlord 3 months notice to vacate.  The standard unfurnished rental is 3 years.  If you like the flat, and are happy to initially accept a year long lease, then whats the problem?  

The lady owner has certainly engaged in some sort of agreement with the french authorities to rent as "furnished" for tax reasons. That's her priviledge then to propose one and only rental contract. 

I strongly beleve there is no other alternative than accepting or quit for another flat (if any matches that one!)