Mothers always seem to know and Tom's mother laughed when an interviewer asked her about Tom giving her grandchildren. Yes, there are 'camp' heterosexuals, but very few and far between and often it is part of a comedy routine and not something they are prone to do in their private life.
If I were an effeminate and 'camp' heterosexual I would 'butch up' to stop people assuming I'm gay all the time.
But as you say we must accept someone's answer as the truth.
I haven't heard or seen what his mother said, but she may have laughed, that someone was actually suggesting that his 19 year old son (who might not be in a steady, strong relationship) would start making kids, when he is at the high
of his career.
Many times people can't actually help how they are. It is in their nature. And to "butch up" just to get away from gay rumors sounds actually awful. Changing yourself to please others is just a depressing thought.