You are probably right. It is risky. But most people including myself do it because it feels good.
Listerine original contains alcohol. If the WG has open sores in her mouth, using Listerine will sting. If she shrieks, stay away. She knows that the same thing goes for you and your dick.
According to the
Listerine website, it kills up to 78.7% of bacteria that cause bad breath. That is not even 100% of the bacteria that it is targeting. STD bacteria are not tested because Listerine does not market themselves as a STD prevention method. I would guess that there is some impact, but much much less than 78%.
Listerine is an anti bacterial. It is not an anti-viral. So it will have little impact on any viruses.