Rules,
I"m in the camp that says you are not being unreasonable at all and love the gym membership idea. At a time that is not emotionally charged, explain your decisions about how you run your house and why, and that it's not up for negotiation. The water and septic are not free when you consider amortization of the cost of installation and maintenance, regardless of how often it needs it. Also, watch out that you don't get taken advantage of. I know of a small a family, (both parents in early 30's and son almost 10), who moved in with the in-laws for 18 months to "help them get back on their feet". It didn't help. At the end of the 18 months, they hadn't saved a dime and couldn't explain why!
My youngest has moved home twice. Both times with the understanding that it's my house, my rules. I've take in my sister, a friend when he got out of the army, a different friend moving back to LA from out of state, etc. When when they're paying rent; my house, my rules. Period.