Desire naturally ebbs and flows in any long-term relationship. I don't see anything to worry about.
And might I add, I often use the phrase "fuck [someone's] brains out" and people tease me about it, so I had to laugh when I read your post.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein