I have had this problem since I was in middle school. It started at the same time that I started being bullied. The bullies were very homophobic and they called me homophobic slurs constantly. I don't have a problem with homosexuality, but I was afraid of being gay at the time solely because of how bad the bullying was and I thought if it turned out I really was gay the bullying would get a thousand times worse. The bullies at my school were horrible and it really traumatized me.
I developed HOCD from being trapped in that hostile bigoted environment for so long, but it went away gradually as the bullying subsided and I made friends and became more secure about my sexuality.
However my Pure-O OCD sprawled out into other sexual based fears like I became afraid that I was a pedophile, that I was attracted to animals, and developed an intense fear of having sexual attraction to family members. Some of these went away when I finally started treatment for my mental disorders at the end of high school, but the ONE pure-O fear that keeps coming back and haunting me is the incest one. It really sucks because I can hardly look at my dad and can't hug him for too long either because I so worried I am going to loose control and become overwhelmed by desire and do things that I don't even want to write down because they disturb me so much, ugh. I have also had nightmares about it. I could shake the fear if I didn't also have this weird physical involuntary "groinal response". That part really scares me.
I go through the thoughts at least once a week after seeing my dad, the whole "what if" question loop. I do not have fantasies about my dad, I try to reassure myself nothing like that is going to happen, but then I get those uncomfortable unwanted feelings and then the whole thing starts up again. So frustrating, I don't know how to shake it. I'm into my twenties and am wondering if anyone else has this specific fear. If anyone has advice to give I would be happy to hear it.