Why shouldn't 'family' money be spent on hobbies? If all money is expected to become family money, how else is someone supposed to fund their hobby? Are people expected to give up hobbies when they have partners/children?
The OP says that they are financially comfortable, so even if the guitar was bought with family money, what is the issue??
The unreasonable response to the guitar is purely greed on the OPs part. She was perfectly happy with her ring before - will all future purchases need to be exactly equal to stop her sulking??
As I mentioned in my post, my DP has massively expensive hobbies and buys equipment for them any chance he can. I love to read, so order far too many books from Amazon - my book collection is probably 100 times bigger than my DP's board collection but cost a fraction of the price.
Should I spent 1000's of pounds on something else to even out the spending?
If we have a joint hobby budget of £500 per month and I only spend £50 on books, why do I need an equal share of the £500 when the remaining £450 would enable my DP to go on a surf trip or buy some piece of equipment to enable him to enjoy his hobby?
I get far more enjoyment from a book than i would a snowboard. Money doesn't always need to be divided equally for 2 adults to gain equal enjoyment from spending it.