I know the pictures we see of Henry in later life are of a large man, but frankly in real terms this was 'later life'. He was married to Catherine of Arragon for 33 years, so by the time he was frolicking with Anne Boleyn he was no spring chicken.
And no, I've never thought Anne was particularly vampish. I think she might have been a touch shrewish, but she held out for marriage before sex, unlike her sister who tumbled into the sack with Henry as soon as she was asked.
Mind you, by then I think Catherine had become so holy that he rarely if ever got any leg-over with her, and Anne was undoubtedly young and pretty. Lord, wouldn't it be lovely to go back and be a fly on the wall, to find out what really happened between them all.
One thing for sure, I'm certain they did more than have sex - and you wouldn't have thought so at the beginning of this series. Dunno what it's like now because I simply don't watch it.