Once you tell someone you’re “not ready for a relationship”, the only standard you get to hold them to is whether or not they add value to your life as a friend, just as your only obligation to them is to be one in return – if the two of you decide to continue any kind of relationship at all.
Their openness to a relationship outside of you at that point is no longer any of your business, at all; and it doesn’t make them desperate or disloyal just because you’re the one that isn’t open to that. It makes them human.
If someone continues a relationship with you in any context after you’ve told them you’re not ready for commitment, it indicates they value you as a person and a part of their life regardless. Don’t then disrespect them by attacking their character because of where you are in terms of wanting a relationship; and if your feelings about it change, it’s on you to communicate that.
It’s also on you to not make your mixed feelings about it their problem. If you’ve told someone you’re not ready for a relationship, and then find yourself feeling upset when they see other people, you need to keep it to yourself until you decide which side of the fence you want to be on. If you decide to stand on not being ready, distance yourself from that person until you can be happy for them when they find someone they connect with. Don’t send mixed signals that’ll only result in hurt feelings by asking for something you’re unwilling to reciprocate. That’s just manipulative and cruel.
It’s not complicated – be respectful and be accountable for your own feelings, like an adult.