Greetings @stephanie58!
That’s a great question 
You’re right that in Wicca the standard deities are the God and Goddess, but I don’t believe there would be anything preventing you from worshipping just one if only one called to you. That being said, I am an Eclectic Witch (as opposed to purely Wiccan), so perhaps someone who is Wiccan will have better advice for you on the matter 
From an Eclectic Witchcraft point of view, the only thing that dictates what is “right” or “wrong” in your practice is your own beliefs! I would set aside any worries about standards in the practice and focus solely on what is right for your situation.
It sounds to me like there is some clash happening between the desires of your deity and your own personal desires.
You might want to ask yourself → Do you want to worship a male deity? Would you be open to giving it a try?
If you are open to giving it a try, I would suggest consulting with Demeter to see if there is a male deity She wishes you to connect with. Zeus comes immediately to mind, but you could explore the Greek Pantheon or even other Pantheons to see if anyone calls to you.
If you are firmly against worshiping a male deity and Demeter is requiring that of you, then you might consult with Demeter and honestly express why you wish to only worship Her. From there, you could potentially continue with what you have been doing, or it could signal that it may be time to express your gratitude and part ways.
As always, these are just my thoughts and suggestions offered up in hopes of helping you find your way forward. Whatever you choose to do, I am wishing you all the best, Stephanie!
Good luck and blessed be! 