The main site is fine because you have to have a subscription and be logged in to see any nudity or sexual activity - the 'join now' journey has been sanitised so there's no actual porn on it (I assume that's UK-only) because that's what I see if I log out and follow that route. Providing a valid payment method is currently accepted for Online Safety Act purposes as verification that you're not a child.
Presumably this is a brute-force action to ensure compliance while they work out how to implement AV on the forum. Since anyone can sign up for the forum without having a subscription to SLR itself, it will require an acceptable form of AV for UK users to be able to see anything here that Nanny would not approve of, and I assume the forum software doesn't have that capability now.
As much as the OSA sucks, the UK is far from alone in this. EU countries will all have to have similar legislation soon because of the Digital Services Directive, and multiple US states already have or are introducing laws that are as bad (or worse) than the OSA. I expect it'll be all of them before too long, except probably California.
If the Project 2025 people (and their equivalents in the UK and elsewhere) get their way, porn will be outlawed entirely. Well, not for the elite, of course, but for the majority of us... no porn for you!