In the example of -- your sister, was it?-- there were two events. Sexual assault followed by a change in orientation.
My view on this, is based on my belief that orientation is voluntary. Therefore, I can not agree that the sexual assault forced her to change. If this seems illogical to you.... well, i'm not sure i have an explanation. It is my belief that I'm expressing. But I've been at this for hours now and i'm getting tired.
If you argue that there must have been an event that caused the PTSD... seeing your buddies blown up, then yes, i would agree that that event caused/forced the PTSD. Again, I can't agree that assault forced a change. Well, if you want to get down to teh nitty gritty... I might agree that the assault forced her to change, but then therapy or just her own thinking/healing absent of external treatment, allowed her to choose to change her orientation back.