No, there is a choice. Claiming there is no choice but to assent to the liberal agenda is being part of that liberal agenda. Refusal of assent is still a reserve power.
Does not matter anyhow, since words and actions are a mismatch here and being for so-called civil partnerships is being for gay marriage.
The structure of the British government does not allow for the queen to do this.
This isnt an absolute monarchy, and hasn’t been so in 300+ years.
If she refuses, it would create a constitutional crisis for them.