Hi, I'd like to help you with your question.
You are correct, it is none of their business if you are dating or involved with anyone or not. You have already answered their questions and the fact that they continue to ask sounds like an issue they have with themselves and is not necessarily about you. Some people are uncomfortable with others being alone because they fear being alone themselves. So they keep asking hoping you will say you are involved so they can feel better. And other people are just interested in gossip. They will ask in a friendly manner hoping you will spill the beans about yourself and give them something to talk about.
The best way to cope with people continually asking you about your personal life is to find a response you are comfortable with then stick to it. Each time they ask you anything related to your love life, repeat your response verbatim and ad nausem. For example, the next time someone makes a comment about your personal life, say "Thank you for your interest, but I am fine." Repeat as necessary. Say it in a friendly but firm tone and smile if you want (to take the edge off). After a while, they may come to understand that they are prying and you are not interested in discussing your love life.
Let me know if I can help in any other way,