Despite trying to follow a balanced path, and considering balance to be quite important, this is a subject I catch quite a bit of flak over. Some folks believe that in order to be balanced, you must worship one God, and one Goddess. If you do otherwise, you're unbalanced, no matter your reasons. (I was once even told I was dangerous for worshiping only a Goddess!) So let's talk balance, and all the different ways it can manifest in a person's life.
Since nature has many lessons for us, I'll start there. Following the Wheel of the Year, we only directly see balance two times - around the Spring and Autumnal Equinoxes. Two days we see that delicate balance of night and day. Two days out of three hundred and sixty-five days. That's it.
In reality though, the Wheel is always balanced when we look at it as a whole. Let's take the Summer Solstice for example. The longest day of the year, the shortest night - doesn't seem very balanced, and on it's own it isn't. However, when we look at the bigger picture we see there is a counterpart we must consider. Directly opposite the Summer Solstice on the Wheel is the Winter Solstice, the longest night and shortest day. Together they form a balance. It's the same all the way around it. At Beltane life is at it's peak, at Samhain death is at the peak - either one alone would not be balanced, but together they create a whole.
On a similar level, while the Northern Hemisphere celebrates Yule, the Southern Hemisphere celebrates Litha. Again there is balance, even if it doesn't jump right out at us. Even if we aren't directly experiencing it at that one point in time, in our one local area.
This can easily be applied to a person's spiritual path. If a person spends a good part of their life worshiping a God in a male dominated religion, then it doesn't seem too unreasonable that for them creating a balance in their life might mean worshiping only a Goddess and focusing on the female mysteries for a time. If we look at one part, it may seem that the person's spiritual path isn't balanced - but when we consider the whole, there is a balance there.
We can also look at the big picture beyond the individual. While we've made quite a bit of progress, society as a whole still doesn't always treat women very well. So when a woman (or a man) finds a safe place to honor the Divine Feminine, it's no wonder that would be embraced. The positive aspects from one helps to balance the negative aspects of the other.
Of course there is much more to consider when forming a relationship with a Deity or Deities. It's not all about balance. Sometimes a Deity brings qualities we lack (which is bringing a balance in it's own way), sometimes They aid us the things we are passionate about, sometimes They bring a lesson to learn, sometimes it's about connecting to an ancestral tradition. There are countless reasons and things to consider! So if a person is a devotee of two Gods, or two Goddesses and a God, or just one Deity, or any other combination - well, so what? That's just the way things work out sometimes, and really, what does it matter? It may not appear balanced, but maybe it is when you look deeper. Maybe there are other reasons. We can't always tell when it comes to the spirituality of another.
Balance can enter into a path in many ways, and it is a very complex topic. Balance will not be exactly the same for any two people, because no two people will be exactly alike. What is balanced for one may not be for another. Even very similar people, on very similar paths likely won't be exactly the same. Trying to force one idea of balance on to everyone could never be truly successful, it's something we all have to work out for ourselves, should we choose to do so.