Boy am I having a tough time wrapping my mind around what a "relationship fact" is. Don't get me wrong this is not an indictment of you or your question, I truely do understand the concept. Maybe this is because of my close connection to GEDCOM or maybe I'm dense.
In the GEDCOM standard when a BIRT (birth) event is entered one of the datapoints that can be entered is a FAMily connection, this is to say, "This person was born on this date, in this place, into this family and here is the source citation for this event."
FTM does not support this piece of information. It relies on a separate piece of information that indicates that the individual is biologically connected to a set of parents.
The same is also true for the ADOPtion "fact". The GEDCOM standard contains an additional datapoint that connects the adoption event to the FAMily that adopted the individual and indicates which parent (or both) was the adoptive parent in the family. In a sentence, "This person was adopted on this date, in this place, by the adult(s) in this family and here is the source citation for this event."
Again, FTM does not support this information in the ADOP "fact" and relies on a datapoint outside of this event.
FTM could and should do a better job of supporting these concepts.