Take it from me, forcing labels on your children fucks them up royaly. You will never be able to undo this kind of damage. Take it from me, if your four year old son wants to wear red lipstick so he can look like mommy or put on some ruby red slippers so he can look like Dorthy, let him. When you see your pre-teen daughter looking at girls, don't go up to her and tell her not to become a lesbian. Chances are, with men being such bastards, she'll be better off with a chick anyway.
My point is, no matter how much control you think you have over your children and influencing their decisions, you have absolutely no control over this. Just face facts and give up. Sure, you can choose to continue, dropping them little hints and clues that mommy and daddy want you to grow up strait. But you are only hurting them and yourselves. You cannot imagine the self loathing that they feel when the people who are supposed to love them unconditionally tell them, subtly or strait out, that they will not be loved the same if their sexual orientation is something other than expected.
You have to understand they can't help themselves or the way they relate sexually to other people, and when to try to make them think that they can, it just makes them hate themselves when they can't.
I guess it all comes down to how much of your child's mental sanity are you willing to risk to try to make them into what you want?