The United Church of Canada is a creature of statute and as such must not be found inconsistent with The United Church of Canada Act

That nothing in this Act contained shall be deemed to limit the independent and exclusive right and power of The United Church to legislate in all matters concerning its doctrine, worship, discipline and government, including therein the right and power from time to time to frame, adopt, alter, change, add to or modify its laws, subordinate standards and formulas and to determine and declare the same or any of them, but subject to the conditions and safeguards in that behalf contained in the Basis of Union. [article 28(b); also The Manual 2010 3(b), p. 7];

which provision references conditions and safeguards, and respecting membership specifically

that no terms of admission to full membership shall be described other than those laid down in the New Testament [basis article 24(2)(a) renumbered 8.6.2(1)(b); note the condition references the text of the new testament not aberrant practices within the early church];

the relevant sections of the new testament directly respecting membership include

"Therefore my judgment is that we should not trouble those of the Gentiles who turn to God, but should write to them to abstain from the pollutions of idols and from unchastity and from what is strangled and from blood." [act 15:19-20];

"For it has seemed good to the Holy Spirit and to us to lay upon you no greater burden than these necessary things: that you abstain from what has been sacrificed to idols and from blood and from what is strangled and from unchastity. If you keep yourselves from these, you will do well. Farewell." [act 15:28-29];

"But as for the Gentiles who have believed, we have sent a letter with our judgment that they should abstain from what has been sacrificed to idols and from blood and from what is strangled and from unchastity." [act 21:25];

where, irrespective of how marriage is interpreted, the new testament terms of admission to membership did not accept sexual relations outside of a faithful committed relationship.

What is being contemplated by presbytery includes affirming sexual relations outside of a faithful committed relationship.