Solids in the sewage end up on beaches because a lot of the more Victorian bits of the sewage systems of coastal towns - which still work just fine - were built for much smaller towns than they're currently under, with a lot less asphalt and concrete.
So, said sewers are generally a lot fuller, and when it rains, less of the rain soaks away and more goes down the drains.
Sewers are designed to handle this, though. There are overflow sections to handle storm surges, and it's ony when those get overwhelmed that the sea outflows get used to deal with the excess. Alas, what goes out the sea outflow tends to be the stuff that floats to the top...
Things used to be a LOT worse when the only overflow sections were the late victorian ones. In recent years a lot of coastal towns have added deep interceptor sewers. I got a look inside Brighton's just after it was built. 6 metres around and a few kilometres long. It'll be a good rainstorm that overwhelms that bastard, I reckon.
Anyway, tampons, towels and wetwipes are still a pain in the toches even if they make it all the way to the treatment plant without causing a Faecal Volcano in someone's garden/bathroom/favourite seafishing spot. All got to be screened out.