Money, at least in the US, is Legal Tender by definition.  This means you can offer it in payment for any agreed upon debt, public or private and it MUST be accepted as payment.  If you offer a payment in dollars, the other party can NOT demand payment in Rubles, Bit coins, Yen, gold, silver, etc...
Bitcoins acceptance as payment for a debt is entirely voluntary.
If you offer payment in Bitcoin to pay.. say your Utility bill... they can, legally, reject that payment. Most likely they will reject it, BTW.