Using the example sentence in the title, would you use the plural linking verb
are or singular linking verb
The same question can be applied to anything with the following format:
(action verb ending with -ing) (prepositional phrase) (singular or plural linking verb) (adjective)
To take a step further, here are my reasoning for the two in the title.
on couchesis boring.
And the other example:
Lying oncouches are boring.