You know, I'm willing to believe that Gaetz genuinely didn't know the girl's age the first time they had sex. The report acknowledges as much. And I'm unwilling to accuse anybody, even someone as detestable as Gaetz, of something that they didn't do especially when there's evidence supporting their claim. And right now, there is plenty of evidence that he did not know she was underage when they had sex.
The first time.
Then he went back for seconds. Anyone who has ever been in an FWB relationship knows how it works. You don't just sit there and make an appointment for every Tuesday afternoon to just get together and quietly fuck like you're in a clinical trial or something. You don't get into an FWB relationship without getting to know someone. There's going to be small talk involved. Playful banter. Genuine conversation. And at some point in that conversation, age is going to come into play. Even if she doesn't directly say "I'm 17!!!", she's going to mention how much she hates the classes she's taking. Or her plans after she graduates. Or what she plans on being when she grows up. Or the fact she still lives at home. Something. Something is going to give away her age, or at the very least cause a reasonable person to at least start asking questions like "So what college do you go to?" to attempt to figure out how old she is.
And even if she's just being paid by an intermediary or something, small talk is still going to be involved. Even the most highly paid prostitute is going to engage in casual conversation to break the ice.
However it ended up, if it just happened once, I'd be willing to bet he genuinely didn't know her age. Then he went back for seconds. At that point, any reasonable person is going to believe he knew, and just didn't care.