I'm not going to let my bf sleep with another girl outside of our agreed arrangement
What will happen if he sleeps with another girl?
He better not or he will suffer the consequences for this!
It is understandable that you don't want your boyfriend to sleep with another girl. Faithfulness is a key part of relationships and many relationships can end due to people being unfaithful.  I also must ask what arrangement do you have?
It seems like you are supporting his action.
Not at all, there should be no cheating the relationship.  I would have to ask you, what was the arrangement and was it something that was made for both of you to honor or accepted by you out of desperation to keep him.