Private - your choice whether or not you choose to use it - but a number of years after you joined Geni, Geni did implement labelling parents as "Biological", "Adoptive", or "Foster" -
Go to your Profile, click "Relationships" and look. - after the names of your parents, there is a Tab with a Karat that probably says "Biological" - if you click on the Tab you get those 3 choices.
By having left it as "Biological" you did in fact tell the system they were biological.
However, last I heard, it was counting paths thru parents labelled "adoptive" as being blood-related, so yes, a bit stupid.
Hopefully you can enjoy Geni despite this.