I will say that "humidity" does something to my wife and me that make us both "unplayable"...I think that's where the analogy falls apart, because I've never had a guitar go mental on me after a period of time.
Probably closer to reality would be that I had a guitar that fit/felt/sounded/responded great, but then I changed with what I expected out of a guitar.
So, considering that neither person in a relationship represents a guitar, it's absolutely worth considering that marriage is a huge step. So getting back to the analogy, it's probably worthy of some thought as to what each person's expectations are, and if they foresee them changing, and due to what circumstances those might be.