I thought this is what the church teaches?
In the UK, it doesn’t matter to many people, including many Catholics, what the Church teaches, or not. Your statement will be taken as your own personal opinion.
Trying to pass of the responsibility for it onto the Church isn’t going to work in the UK, or any other Western country anymore. Even in the US, that excuse is way past it’s prime.
You asked how you, in the UK, could publicly say that you oppose gay couples as foster parents without being accused of being motivated by personal antipathy against LGBT people.
Basically, you can’t. That is precisely what by far most people are going to assume what you motivation is, and it is going to be very, very difficult to convince them otherwise.
Expecting people to indulge and tolerate your personal sentiments because they are “Church teaching” is unrealistic in the UK and by far most other Western societies at this point in time.
I get the impression that you are looking for some type of magic fairy dust that you can sprinkle on your argument so that you are not accused of being motivated by antipathy against LGBT people by the general public. Sorry, but magic fairy dust does not exist.
It IS true that appealing to religious beliefs used to work, years ago. It just doesn’t work at all any more.