The question is, what really are our obligations. The EU wants continued contribution to that which we were involved in as members, but being an ex-member is a different situation.
Staying part of and continuing to contribute to projects that benefit us isn't so bad, but continuing to pay for that which is part of a club to which we no longer belong, and which the contract didn't stipulate continuing to contrnute after leaving, isn't reasonable.
We are leaving anyway IF our politicians are to be believed, so 'flit' doesn't come into it. These negotiations are about future trade relationships and saying, "No", to bad deals and leaving without agreement is perfectly honorable.
What isn't honorable is demanding unreasonable criteria that would leave us in the EU in all but name, as the EU would continue to make rules we'd be forced to obey. The sensible conclusion from the "discussions" so far is that the EU should be the party untrusted in the future.
The UK, on the other hand, is likely to be seen as the weak push-over in the future, given May's record so far. That might change slightly if we feel forced to go for no deal.