The probability that "extrafamilial sexual abuse" describes also some wanted sexual experiences is higher compared with intrafamilial abuse. There is a lower power difference. This suggests the hypothesis that in the average the impact of "extrafamilial sexual abuse" should be lower.
There is indeed a lot of research which indicates that incest has much more negative impact.
Ligezinska et.al. 1996 consider 41 children with "extrafamilial sexual abuse". They find "that the children's perceptions of self-blame and guilt for the abuse and the extent of traumatization predicted their self-reported symptomatology of depression, social efficacy, and general and abuse-related fears. As well, child's gender predicted the level of general fearfulness. None of the other demographic or abuse-related variables were related to children's functioning."