You sense that the enquiry doesn't feel right. If you still say "yes", you are saying "no" to yourself.

We should all be in good contact with our circle of non-negotiables! What is ok, where does it start to get difficult? And what is not possible for me?

The non-negotiable is not fixed. It is allowed to change, it is allowed to struggle for it. This is even important, otherwise it becomes fundamentalist. It is not uncommon to only realise that the red line has been crossed when it has already happened. This is usually not a disaster either. The important thing is simply to realise that this is the case and to look for ways to get out (or not to make the same mistake next time). After the mistake is before the next mistake.

Say "no" to the enquiry from time to time - and  "yes" to yourself.

Sources and further links

A very good way to communicate your non-negotiables is tit for tat.  

Rolf Dobelli describes a similar concept as the "circle of dignity".