>we pronouce rain as though it were spelled "rayn"<

Max, try it a different way. Pronounce 'rayn', then cut the consonants off each end and pronounce the 'ay' sound you have in the middle. Do you pronounce this without moving your tongue or changing the shape of your mouth?

Changing the shape of your mouth or moving your tongue is in fact making another sound. I have to do this to say 'rayn'. That makes it a diphthong, at least for me. Compare this to the vowel sounds in 'cat' or 'book' or 'read' (either tense!). I can easily say all these with no movement - they are 'monophthongs'. (Is that a real word or was it made up for this discussion? It is truly wonderful!)