It's important to remember that every individual and every relationship is unique, so there could be various reasons why a married person might not want you to date someone else. Here are some possible reasons:
Jealousy or possessiveness: The married person may feel jealous or possessive, fearing that they might lose your attention or emotional connection if you start dating someone else.
Emotional attachment: They might have developed feelings for you and don't want to see you with someone else because it could be emotionally painful for them.
Insecurity: The person might be insecure about their own relationship and feel threatened by the idea of you dating someone else.
Fear of losing the friendship: If you're close friends with the married person, they might worry that a romantic relationship with someone else could change your dynamic or jeopardize the friendship.
Moral or ethical concerns: Some people may have strong beliefs about monogamy or may be in an open relationship where certain boundaries need to be respected.
Overprotectiveness: They might be concerned about your well-being and feel that dating someone new could lead to negative consequences or hurt.
Misunderstanding: There could be a misunderstanding or miscommunication, and the person may not have meant to prevent you from dating someone else.
It's important to have open and honest communication with the married person to understand their reasons better. It's also essential to respect their feelings and boundaries while considering your own desires and needs. Ultimately, you have the right to make choices about your own life and relationships, but it's crucial to handle the situation with empathy and understanding. If the situation becomes too complicated or uncomfortable, seeking guidance from a neutral third party or a therapist can be helpful in navigating these emotions and conflicts.