A couple of points that have already been mentioned but need reiterating:
Children in foster care ARE in care. When you say a child is in care, the vast, vast majority of the time this means that they are in foster care. Some may be in children's homes, yes, but foster care IS care. And also, being in foster care does not automatically make you feel loved and wanted (especially if you're gay and with homophobes).
As grandhighpoohba said, many people do know that they are gay at a young age. And in any case, it doesn't matter what age someone realises they are gay: when they do realise, they are going to interpret this according to the messages that have received PRIOR to this point. If they have heard homophobic comments, they are going to be worried about this new development in their feelings. If they have heard these comments in the playground, and, when talking to those caring for them, have received a neutral answer or silence, this is going to give the message that the homophobia they have encountered isn't something the adults feel it is necessary to tackle. Or, worse, that the adults actually agree with it but are deliberately maintaining a 'neutral' stance.
On this point: just ask anyone who suffered (like myself) under section 28 whether a stance of silence and/or neutrality actually does anything but harm a gay person when the homophobia around them is very vocal. All it does is say that the adult does not care, or that they do not care enough to stand up for said gay people. You cannot have vocal homophobia on one side and neutrality on the other. Therefore it is important that foster parents can say that it's OK, and not just remain neutral.
On private families: private families are different. At the end of the day, with private families it is a negotiation between the prerogatives of the parents to bring their children up as they wish and the best interests of the child. And, as it should be, parents have the right to bring up their own children as they wish. When the state is bringing up children, however, it needs to do so giving them all the protection of the law. But foster parents - and potential foster parents - do not actually have any right over the children in their care, so the situation is different.
Basically, children in care DO care about minority rights because many of them ARE minorities! And the state, as parent over these children, has the responsibility to look after their rights FIRST. And as I have said, and I think others have said, I also think that foster parents unable to support a foster child's religion, and who, being vehemently anti-religion, could not tell them that religion was OK (even if they did not speak strongly about it), should also not be allowed to foster. It's about being able to support someone else's child in being or doing something that is OK according to the law - and not to do with how these things are viewed according to the morals of the person doing the fostering.