I was 16, my 20 yo boyfriend was a lovely bloke who treated me well. My parents hated him being older & didn't like him (he was into theatre & music, not rugby & sailing) He made it very clear that whilst he very much wanted to have sex with me, that he wanted to wait until I was sure it was what I wanted to. When we did, it was very much me pushing it. He was understanding about some shit going on in my life & I felt he was the only one who understood me. He put up with me hanging out with my ex boyfriend in the school holidays while he was at work, he got me a job at his work when I wanted to work in the holidays etc. His mum & I got on really well.
I partied with my school friends, went to dinner with hus friends, he was lovely. We ended up being together 10 years, until he decided that actually he really didn't want children. He though he did & had said he did, but realised what he wanted to do with his life (travel to remote places and be heavily involved in theatre) wasn't fair on children & he didn't want to compromise.
I was heart broken, he was heart broken. It was awful, but I wouldn't swap the 10 year relationship I had with him for anything.
Just because he's 20 & she's 17, doesn't mean he's going to be a bad influence in her life.
Your older DD is being ridiculous.