Slowly, they work on it, going through some lighter scening that lets Frank get used to trusting Matt to tell him when he needs him to stop and, more importantly, trusting himself to stop.
And then something like this happens: http://archiveofourown.org/works/4536168/chapters/10558566
Re: Matt/Frank Castle, BDSM Fail
And then something like this happens: http://archiveofourown.org/works/4536168/chapters/10558566
Whoops, did someone say angst?
*vanishes in a cloud of trash*