That's kind of my point. Catherine has never said that TLR is Ryan's dad. She's always said the opposite, that he isn't. There's never been a dna test.
Remember back in season one, TLR is fresh out of prison, at his mum's house and his mum said "Becky had a kid--is it yours?" Next thing you know, TLR is stalking the school and sees Catherine collecting Ryan and commences stalking. It all escalated from there.
I think gang rape or pimping would cover why Catherine has been so insistent that TLR isn't Ryan's dad, as well as the "things she's never told anyone" that contributed to Becky's suicide.