Benson, in context the words "think" and "know" have -- for me, anyway -- different connotations.
"I thought our love would fill the earth..." tells me the singer believed that at the moment he lay with her; it has a sort of bittersweet quality to it that speaks of that moment of first being wildly in love with someone,
Whereas "I knew our love would fill the earth..." conveys a certainty that this love will always last this way(to the end of time); something that is not true in my experience.
You think it's going to but it doesn't. It changes into something else, or it dies. I like this song because it says something true; changing the word to "knew" makes the song false.
Leaving out both "thought" and "knew" doesn't bother me, I like what it does, except it spoils the meter.(grin).
Blessings,
Barbara