I'm a guy in a committed relationship but secretly would love to be somebody's loving wife
Why do you want to be someone's wife even though you're a man?
I've been in a relationship for the last 2 years and it's time for me to get married.
You may have some sort of body dysmorphia, or maybe you're realizing that you don't want to be in your relationship anymore. Both of those things are fine, but people would expect you to speak with your girlfriend about it.
It's just a fantasy. We are going to grow old together and maybe adopt a child.
That's fair if it is just a fantasy, but maybe if you tell it to her she would help you through it? That's what a good relationship is for!