Total Strangers, without a doubt. I know who the family members, friends, teachers, and children they come in contact with are. I can do the appropriate checks on them and make sure they're decent people. It's the unexpected interactions that concern me. The strangers and random interactions. The people who you cannot predict them meeting and talking to. Those are the ones that concern me.
The biggest threat to my grandkids are those who intend to do them harm or are so careless and inattentive they cause harm without thought. There's no one in the family like that, gay or straight, so I suppose total strangers is my answer. But adding the sexuality component, well, it just doesn't fit here.
Statistically it is family/household members I think I read somewhere, but that does not mean stranger danger should be taken lightly.
The biggest threat to kids, IMO, are people who think to impart their thoughts and biases on impressionable minds without consent of the parents.
Building block or stumbling block.... choose.