I’ve already outlined in another comment why they could increase transmission.
They increase net volume of air exchanged, therefore all else equal they increase the release of aerosols. There’s furthermore a strong chance that they actively nebulize respiratory droplets into fine aerosols; ie they incresse the relative aerosolization and therefore the absolute aerosols.
They make convo partners stand closer together or speak louder.
They alter risk appraisal.
It is foolish to rule out any downsides. At best you’re advocating for a condom full of holes. At worse you’re advocating for a condom that magically worsens STI transmission.