Be honest. Maybe something like
"At this point in time, this is a V shape thing. I am not ready/willing to enter into romanceship with you and make it be a triad thing.
Please keep your behavior towards me in the friend bucket. These behaviors are fine (list). These behaviors I am not comfortable with (list.)
I do not see my feelings on triad changing in the near future. At this point in time my feelings seem 100% on it being a V and only a V.
We could check in if you like one year from now. To see if yours, mine, or Hinge GF feelings have changed or remain the same on the poly configuration we share. But for now, please accept it is a V. Please respect my boundaries."