...he says he has grandchildren and they would have been arrested when he was a kid. OK, based on this statement I am presuming they are possibly at least in their early teens. This would put Ian in his late 50's early 60's if he had his children when he was around 20 and they had theirs at the same age. Forty years ago homosexuality was still a taboo subject frowned upon by society and punished by the authorities if 'openly displayed ' in public. Just 'looking' gay would make you the object of ridicule and harassment and I think this is what he is trying to convey.
No, I still think you must be wrong, because he said they would have been "arrested". He didn't say "criticised". So he must mean that his grandchildren perform sexual acts with other people of the same sex.
He must mean they are boys, not girls, as Lesbianism has never been a criminal act in England or Wales.
And (if English or Welsh) he must mean they do it in public, because sexual acts between two adult males, with no other people present, were made legal in England and Wales in 1967.
I suppose another possibility is that what he said was not true.