Strikes me "forgotten about it" could mean no more than "It wasn't in the repertoire any more and rarely came up in conversation".

But to muddy the waters further, in a review of the Flack and Presley versions for Folk Review in 1972, Karl Dallas asserted: "[it's a long time] since he wrote First Time Ever as an illustration of the fact that it ought to be possible to write a commercial lovesong without descending into drippy inanities".

Which suggests (if KD has his facts right) that it wasn't so much a spontaneous outpouring of ardour (cf. lovesickness and whiskey) as a conscious, even intellectual, act of creation. Whether he managed to avoid drippy inanities is, of course, an open question.

And for what it's worth, I love the song.