If I may expand on your mention of the biblical symbolism, the rainbow is also a sign of fertility. After the biblical flood, God reiterates to the survivors His original command to be fruitful and multiply, as He did to Adam and Eve. It is in this context that He establishes the biblical rainbow promise.
On an even more fundamental level than biblical imagery, rainbows form in sunny weather after the sky has watered the earth, leading to future plant growth. Most ancient cultures ascribed masculinity to the heavens and femininity to the earth, and thus drew a clear analogy for the union of male and female that produces child.
By all rights, the rainbow is the original symbol of a man and woman building a family and future generations. Using the rainbow as a symbol for barren sexual hedonism and unproductive carnal gratification is nothing short of heretical to the natural order of things.