The first necessary heresy
is, "Why not God
?" The faith-sense of worshipping a Mother is substantially different. We expect nurturing, contact and warmth. She will always have a little something special for us. For me, the experience of God the Mother was so moving that the shackles of Patrifocal religion
dropped away fairly quickly.
It is a small step from having two ways of conceiving God to having two Gods. That's why there is so many energy spent explaining the Trinity in the Christian tradition. It's inherently unstable and the Moslems were wise to drop it. If you're going to hold up monotheism as this paragon of religious evolution, you'd better really be monotheists, and no one does that better than the Islamic traditions. (The Judaic traditions come in a close second, but they still have the historical baggage of Yhwh's transition in the Torah from one God among many, to the one God of the Israelites, to the One and Only God.)
There are several ways to approach polytheism. Another path for me is Jung. There is ample evidence that there are non-conscious psychological processes in human beings. To a greater or lesser extent these processes exhibit cognitive faculty, and to a greater or lesser extent some of these processes appear to be autonomous. If our personal psyche is not monolithic, then there is little reason to expect our collective psyche to be monolithic. Thus, spirits can be regarded as a legitimate scientific model of this collective psychology.
A third route which lead me to polytheism is sociopolitical. The women warriors of the last half of the Twentieth century have fought hard to establish a history and tradition of matriarchal authority. In the feminist paradigm, the beneficent Goddess-centered religions of pre-history were obliterated by the rise of patriarchal warrior cults. Whether these primal Goddess cults were monotheistic or not is moot. If these early matrifocal cultures were like the modern indigenous peoples for which we do have good historical records then in the matriarchal golden age, people attended to a wide variety nature spirits as a pantheon. You might hate your neighboring community's Gods but you would not doubt Their existence or Their power. It does not matter to me if there really was a matriarchal golden age, because the ideal of a tolerant, earth-centered society is a worthy paradigm whether or not it has a genuine historical precedent.
The last route for me to polytheism is philosophical. The Gods exist. That statement may merely be tautological. The number "2" exists, but I never ran into Him on the street. There is a sense in which anything we can think of exists, because, certainly, that encoding and representation of information exists. Clearly, there is a spectrum of existence. An atheist might propose that the Gods exist in the same sense that Don Quixote exists. (Interestingly, a pantheist would probably say that Don Quixote exists in the same sense that the Gods exist.) The existence or non-existence of incorporeal beings is only relevant to the extent that these beings are seen to act within the world as we know it. Does Yhwh exist? All we know for certain is that a whole bunch of people act as if He does. Therefore, even the most obsessive materialist can accept the pragmatic criteria of judging religions: there may be no materially manifesting Gods walking across the face of the earth, but, nevertheless, the beliefs and consequent actions of the various adherents to different beliefs do directly impact the material world, and, therefore, belief systems can and should be judged according to that impact. It is precisely in this sense that even strict monotheists refer to Jesus, Buddha and Allah as separate Gods.