It's explained in the novel. Josh asks Chase this exact question and Chase awkwardly replied, "Don't beta males also get wet when they're aroused?" Instead of laughing or clearing up Chase's misunderstanding, Josh realizes Chase's lack of sex education is due to his trauma towards sex. Bc of Chase's rut, his family's lack of understanding, Chase is very adverse towards sex, leading him to block out anything related to it. So he lacks common sense or education about alpha, omega, and beta sex.
Chase was also a virgin prior to sleeping with Josh. Josh is Chase's first and only ever partner. Instead of correcting Chase's misconception, Josh just agrees with Chase bc Chase is already self-conscious and insecure about his lack of sex experience. Josh didn't want to crush what little ego he had.
I see that makes sense, thanks
Josh is self lubricating and Chase still doesn’t realise he’s an omega