God love Wiki. Didn't take long. So much for my theories.
"I Want to Marry a Lighthouse Keeper", Erika Eigen (from the 1969 album of her group, "Sunforest" - movie version is somewhat different from soundtrack)
But I've always wondered where the producers dug up this Erika Eigen... who was she? Did she ever record anything else? Is it a pseudonym for W. C. Williams... perhaps W.C.W. didn't want to be associated with such cheesiness?
Pulpist... have you seen the movie? Plus, I LOVE this song. Used to put it on mixtapes when I was in short pants, right between Crass and Discharge. It's brilliantly crafted to be JUST what it is supposed to be.