Some of them have sons. Some of them were Scouts themselves and want to help bring the program to younger Scouts. And some are exactly what you suspect they all are.
I'm a Scouter. Anybody, gay or straight, who wanted to join up in the BSA without having a Scout in the Troop or Pack would get pretty closely questioned and would have to pass the "smell" test to get accepted. What you see so often in these cases is that the offender (regardless of their sexual orientation or marital state) usually assaults a boy outside of the Scouting environment; they get the kid over to their apartment or house, or take them to a movie, etc. Within Scouting, we do a pretty good job of making sure that we all keep an eye on each other, using the Youth Protection guidelines.