Arrange a time to talk with the other parents. Remain calm, and simply present your point of view and the basis for it. Ask them to explain their decisions so that you can understand where they are coming from. Listen respectfully! You might learn something, or you might be able to teach them. Have an open discussion without judging or making accusations.
If you cannot come to a resolution with the other parents, let them know in a polite manner that you respect their decision, but need to ask them to respect yours. Your child will not be allowed to come to their house (or play/associate with their children, if the situation is severe enough). You would appreciate it if they would let you know if your child is at their house without your permission.
Talk with your child. Explain why you feel strongly about your own choices. Make it clear that you are making these choices to protect your child because you love him. If he is old enough, ask for input about the situation, and come to a solution together – you may have to compromise a little to ensure that you are not setting up a situation where he will be sure to get in trouble. Make it clear what the new rules are, and what the consequences will be for violating these rules.
Follow the basic philosophy that these other parents seem to have – your number one priority is your child’s safety. Whatever you have to do to ensure this safety, DO IT! Do not worry about losing the parents as friends or if your child will be upset with you. If you have a strongly held belief that concerns your child, you need to stick to it.