Exactly right. Further - This hobby should never cut into things for family, IMO. If it does then find another activity or cut back. What you spend on SP's and MPA's shouldn't impact your finances to a point that you think your SO will notice or you have to start socking away here and there for appointments. IMO - it should be entirely from discretionary funds.... YOUR discretionary funds which are nobody's business but yourself.Joint account is the problem.
Bills are paid, from any account... and her money is her business and my money is my business. No explanations required. She has the stability to know though that money will never be an issue. In some cases it could be a stress, I can see that. I'm lucky i guess.