Using the example sentence in the title, would you use the plural linking verb "are" or singular linking verb "is"?
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.
The first option, “Lying on couches is boring”, is correct. The word “lying” is used as a gerund here, meaning that although “lie” is a verb, “lying” is a noun. This is a singular noun, so saying “lying is boring” is just like saying “The book is boring” or “My cat is boring”.
“On couches” is a prepositional phrase, modifying the gerund, so basically it’s an adjective. It’s like saying “The green book is boring”, or “The book about fish is boring”. In both cases, “green” and “about fish” are used as adjectives, and don’t really directly affect the verb.
So it’s “lying on couches is boring” because “lying” is singular, and you would still use “is” even if there was only one couch (“lying on the couch is boring”), as “on couches” only describes the noun/gerund.