You do not LET anyone do anything.
You choose your behavior. Other people choose their behavior.
So speak to your willingness in your behavior. You do not present this as a threat or ultimatum. Just a simple "I am willing/not willing to go here."
- I am willing to be fluid bonded with you when you are fluid bonded to me and no other. Condoms on for all others. Testing required.
- I am willing to be fluid bonded with you and your lover (through you). But no more people than that -- condoms ON the rest of the time and testing.
- I am willing to be fluid bonded with whoever. No condoms required. Testing required / not required?
- I am NOT willing to be fluid bonded with more people than just you. You are free to choose, but if you chose this, then with ME there will be (condoms? No more sex?)
Or some other thing not listed in the examples. But you speak to YOUR willingness. You share your body with her when you share sex. You choose HOW you share your body and when. It is your body and your behavior you are in charge of.
YOU choose what you are willing to do and not willing to do. You can share this information calmly.
Then she can choose her next behavior with full information and tell you what her choice is.
Then you can choose what your next behavior will require next.
Everyone is free to choose their next behavior choice. They are not free of the consequences of the chosen behavior.
If you feel reservations? Stop sex. Or stop sex without protection. Just pause it (no fluid bonding between you)... and sort out the emotion/trust stuff in the heart bucket and mind bucket. THEN come back to the body bucket of health and make decisions about physical health with willing mind, willing heart, and willing spirit. Could be a pause of a few days. Could be longer. Could be a new rule. But proceed with all your healths in mind -- mental, emotional, physical, spiritual. You are responsible for the well-being of YOU -- in mind, body, heart, soul.