Security proof in the Malicious Model
In order for a protocol to be secure, the output of any adversary must be simulatable, including that of an adversary that just outputs its input.

Moreover, note that in this case the protocol instructs that $P_1$ send its input bit $x$ to $P_2$, so in the case where $P_2$ is corrupted, it can output the input of $P_1$, so that must be simulatable as well.