Jesse Bering (Scientific American) writes about some of the research on childhood predictors of adult sexual orientation. If the subject interests you and you're not familiar with the research, it's a quick worthwhile read. Bering ends by asking:
If researchers eventually perfect the forecasting of adult sexual orientation in children, would parents want to know? I can say as a once prehomosexual pipsqueak that some preparation on the part of others would have made it easier on me, rather than constantly fearing rejection or worrying about some careless slipup leading to my “exposure.” It would have at least avoided all those awkward, incessant questions during my teenage years about why I wasn't dating a nice pretty girl (or questions from the nice pretty girl about why I was dating her and rejecting her advances).
I can understand why he might see this as potentially helpful, but I could also imagine some potential downsides, such as placing too much emphasis on the implications of sexual orientation one way or the other. If a parent is highly anxious and not well-attuned to their child, is a prediction really going to be helpful? In any case, while some prediction is possible, I doubt, for several reasons, that we will ever be ever predict adult sexual orientation with certainty.