Why did it matter that Catherine Howard had ''carnal relations'' with Francis Dereham- how was this treasonous?

I know she had to disclose her sexual history before marrying Henry, but what was the problem with her and Dereham's past relationship, how was this seen as treason and why was Francis Dereham taken into torture for it and eventually executed for a past relationship with the Queen? If they didn't find out about Thomas Culpepper, would she have been spared execution?