Don't quote me word for word, but his mother said: "Grandkids from Tom? Hahahaha!"
There is a train of thought that 'campness' in the gay community is something you develop to fit in with your peers.
What her mother said could simply mean, that Tom has clearly said, that he doesn't want kids, ever.
And I don't get the logic of trying to fit in with your peers in Tom's case, as it is suggested, that he is in fact hiding his gayness, in the closet, and does not want to fit in with his peers. If he was openly gay, I would get the train of thought, but he isn't, he says, that he is straight, so..