Blimey AF.
What on odd pov if I may say so. You think that there is a new boundary of faithfulness that comes with marriage, that doesn't apply to long term couples?
I have never heard this from anybody else in my life. From the day that a man is known as a boyfriend to me, and he sees me as his girlfriend, it's exclusive.
By the time we're moving in with each other, discussing the future etc it's absolutely cast iron - no snogging other people.
My DP and I have never even discussed this - we love each other, and we both expect 100% faithfulness and commitment from each other. If my DP was to snog somebody else and then say (after 11 years of living together) that it didn't count as we weren't married, well. You'd hear my reaction from the moon.
My relationship is 100% as strong, unbreakable, important, real etc as anybody else's whether there's a band of gold on my finger or not.