Ooh, an asinine comparison to a natural phenomenon to justify his bigotry! Time to destroy it. WITH SCIENCE!
"The Creator of the Universe has made a gorgeous rainbow. Each color has its own beauty, its own characteristics, its own unique place in creation."
Haha, nope! If you bothered to pay attention in your elementary science classes, you'd know that what we perceive as colour is really just electromagnetic radiation with wavelengths of varying frequencies. And guess what! Those frequencies aren't discrete, they lie along a continuous spectrum and merge into each other. You may identify green and blue as separate colours in the rainbow, but if you look carefully, you'll see a narrow band where they blend together and create teal, which is just as beautiful a colour. So, according to your silly analogy, Jews really ought to intermarry with other groups of people and produce varying, continuous grades of semi-Jew along a spectrum. They'll be just as valid.
"If we mix all the colors of the rainbow we will be left with a single murky, grayish, greenish color"
Actually, you get white. Isn't white often a symbol of purity, something to be desired? (Hence white wedding dresses in the Western world.) So according to your rainbow analogy, here's another reason why Jews and non-Jews should intermarry.
YOUR ANALOGY (and, therefore, half your argument) HAS BEEN DESTROYED!
As for this bit:
"As for me minding my own business; when someone loses an object, we are obligated to return that object to its proper owner. So that lost Jewish boy who was thinking about marrying a non Jewish girl had lost his way, and it was my obligation to return him to his Owner."
People aren't mindless (except in your case) objects to be owned! That's why all civilized societies have done away with slavery, and that's why parents (who are more real and tangible "creators" than the tyrannical sky-pixie that you worship) don't have complete control over their child's life and get to dictate every part of it. So fuck your god, and fuck you.