I get why that particular line sticks with you — it's the kind of lyric that sounds like it should belong to some big wedding ballad. I went digging through the stuff I know and honestly couldn't find a famous, mainstream song that literally uses the exact phrase 'marrying you forever.' The closest big hits people usually mean are songs that say 'I wanna marry you' or talk about 'forever' in the same verse, like Bruno Mars' 'Marry You' (which has the memorable 'I think I wanna marry you') or Train's 'Marry Me' (which leans heavily on 'forever' in its chorus). Those get mixed up a lot in memory.
If I'm honest, a lot of times we remember the sentiment more than the exact words — so a small indie tune, a church hymn, or even a translated pop song could be the origin. If you want, I can walk you through a few quick search tricks I use: Google the phrase in quotes, try lyric sites like 'Genius' or 'Musixmatch', hum the melody into Google/SoundHound, and check wedding playlists on Spotify or YouTube. One of those usually cracks the case for me.