When it comes to sex, there is very little new under the sun. A lot of the sex related topics already exist and/or have been beaten to death, so I guess it comes down to what people come to forums for.
I wasn't aware that the mobile app excluded forum access, but there are an increasing # of people that won't access something if it doesn't have an app. You can of course still access the site via a browser on your phone versus the app, but some will choose an app over that if one is available.
There are topics (like Covid and politics) that have polarized people into extremes on both sides. I know I personally have added a few of those to my Block list, as I'm sure others have done the same to me. I have seen people block others with no dialog at all with them.
Lastly, SLS has made some unpopular choices in the past (like eliminating email) as well as having reliability issues that may have driven some of the current or potential forum goers elsewhere.