An excellent discussion on the issue of cohabitation and spousal support is found in the Byrne v. Byrne decision. The court noted that just because someone starts cohabitation after separation does not in itself lead to termination of spousal support. In this case, the courts did not terminate spousal support simply because of cohabitation.
http://www.canlii.org/en/on/onsc/doc/2016/2016onsc2315/2016onsc2315.html