Why jealousy can happen
Sex, friendship, attention and attachment can overlap. You may feel threatened by another partner even if you intellectually agreed to non-exclusivity.
Do not turn jealousy into control
Feeling jealous does not create a right to monitor, punish or restrict someone outside your agreements. It does create a reason to ask what you now need from the relationship.
Check the underlying expectation
Sometimes jealousy reveals that you actually want exclusivity or commitment. Sometimes it reflects insecurity that does not change your preferred relationship structure. Those lead to different conversations.
Your options
You can continue with clearer boundaries, renegotiate exclusivity if both want it, take space, or end the arrangement. The sustainable option is the one both people can genuinely agree to.
Use changes as conversation prompts
FWB advice often encourages people to decode every behavior: a longer text means attachment, a delayed reply means lost interest, a sleepover means the relationship is becoming serious. Real relationships are noisier than that. Look for sustained patterns, then use those patterns as reasons to ask rather than as proof of an answer. This keeps you from making major decisions based on a single message, busy week or affectionate evening.
Manage the agreement, not the other person
Boundaries work best when they describe your participation rather than trying to control someone else. You can say what information you need, what level of contact works for you, or what would make you step away. You cannot make a healthy casual relationship by turning uncertainty into surveillance or manipulation.
Labels help describe a relationship, but expectations are specific to the people in it. When something important changes, a direct conversation is usually more useful than trying to decode a rule from the internet.