I would tell your parents: We are really in a good place and expect to stay there (or get better) financially. If you want to gift us the $15K because it would make you feel better, or you somehow feel obligated to, I would love to accept it and use it towards my financial goals. But not everything in life is fair, and I'm are just glad I don't need the 15K, so you don't need to even offer it.
This reminds me of my two kids, 5 & 7, that are constantly complaining that one gets more than the other. Like, stuff out of my control, like more days off from school or more birthday parties to attend. I'm so over it. I love and treat them equally, but I'm not going to weigh their desserts so they get the exact same amount. Or give them some sort of cash balance if one of them gets a christmas gift worth more than what the other got. They have to learn to deal with little inequities. And I think 15K is a little inequity in this situation.
Another thought, if you have kids, or plan to have them, putting the $$ in college savings for them might be something that will feel less like you taking it, if you end up accepting the money.