If it's being asked by an unmarried man, reply, "If that's a proposal, I accept!" Then turn to your other friends and announce your engagement.
If it's asked by anyone else, you could say:
- I'll answer that if you'll tell me how much money you make and how much you owe
- I'll answer that if you'll tell me why it's any of your business.
- I have so many boyfriends, I'm having a hard time deciding which one to propose to.
I don't think there's anything to joke about. This IS a serious problem for single people. Somehow, being unmarried, we are acceptable targets for rude, insensitive, and inappropriate comments and questions.
Nip it in the bud.