There is no real evidence that Catherine Howard had sex with Thomas Culpeper.
She was, from all evidence, a silly teenager who was brought up rather neglected and barely literate.
Even Cromwell found her state during her imprisonment pitiable.
She was a teenage girl facing beheading. I can't imagine anyone would find that an appealing prospect and yet, from all historical counts, she was composed at the time of her death.
She did indeed have a rather long sexual relationship with Francis Dereham, and quite honestly, she should have admitted to being pre-contracted to him as then Henry could have banished her. There was pleny of hard evidence to back that up.
But she was rather foolish, young and very upset.
Personally, I always felt she was rather mistreated by history.
She wasn't old enough, educated enough or socially skilled enough to be truly 'wanton' in the same sense as Ann Bolelyn.
She was used as a pawn by her family, too.