The daily SSRI is being run at a low dose, because the length of use is indefinite. The rat gets the benefits needed from it, but when it comes to sexual performance, it isn't a large enough dose to make a significant difference.
Not to mention that different SSRI's react differently to individuals body chemistry.
The rat has been on a different SSRI at a comparable dose, and performance was vastly improved, but side effects were too harsh.