I always liked the song but even back in the day I remember thinking "what the hell am I singing about" as the tune played on the radio.
I found this article on "song facts"
With the famous "cake out in the rain," this is one of the more lyrically-intriguing song ever recorded. Jimmy Webb, who wrote the song, explained in Q magazine: "It's clearly about a love affair ending, and the person singing it is using the cake and the rain as a metaphor for that. OK, it may be far out there, and a bit incomprehensible, but I wrote the song at a time in the late 1960s when surrealistic lyrics were the order of the day." Yep...he was high!
I "guess" cake out in the rain "could" refer to the relationship, leaving it out in the rain and it fizzled out. But, it's sort of a roundabout way of getting to the point. LOL
TIS
http://www.songfacts.com/detail.php?id=1875