Why ever enter any bilateral agreement? Any agreement surely has some extra benefit that is derived from that agreement which is not otherwise available as a nation being on her own?
If benefit/s can be gained by any nation forging an agreement with one other country, then by virtue of pure logic be also available between three nations!
Etc etc etc.,
It is immaterial whether membership to a body is comprised of 2 or 27 parts if rules are adhered to and followed by others.
Issue then becomes when a nation within it self can not agree to the benefits derived from that membership.
There you have Brexit.
Common sense is it not?