Quoted from commissioningsupport.org (can't work out how to link a pdf) but feel free to google. It's Research Report DCSF-RW023 from the department of children, schools and families.
"The profile of children in residential care:
? Approximately 6,600 children are in residential care in England (around 11 percent of all
looked after children), most of whom are over 13 years old;"
The children who would be potentially fostered by this couple are hugely unlikely to end up in residential care, so that argument is a red herring.
Fostering is a job. This couple can believe what they want, but they have admitted that they are unwilling to undertake a fundamental aspect of the job - to offer acceptance and understanding to a troubled child. Would you expect a job at McDonalds if you refused to serve meat?
BTW, comparing someone's sexuality, which causes no harm to anyone, and is nowadays firmly established as not a choice, to criminal behaviour may be why some people here feel quite offended.