Definitely. That's one of the most common things in most services' ToSes. From a business point of view, carrying that content probably causes much more hassle than it's worth for most companies that aren't set up for the adult entertainment industry. If and when a customer accidentally sends porn to a 13 year old, which would you rather tell the parent screaming at you: "we're sorry and we'll instruct the customer not to do that again" or "oh my, that's against our policies and we'll be sure to cancel their account. Thank you and please don't organize a pitchforked mob to march on our headquarters".

I don't care if adults look at porn or engage in porn-based commerce; I'm not saying "porn is immoral and it's good to block it!" I do believe that the vast majority of businesses not already working with that industry don't want the aggravation that comes along with dealing with that content, though, and that's why it's widely barred.

Maybe I'm unclear on what Mailchimp does. Do they host private mailing lists, or just do outbound 'opt-in-but-not-really' marketing?

They host (mostly) legit bulk email. So if you subscribe to a newsletter or get announcements about sales from a web page via email, it's likely MailChimp

The newsletter part is what I was driving at. If they host that sort of thing, then are they lumping in private newsletters or mailing lists for sexual abuse therapists (or survivors) with unabashed pornography?

This is just one reason why people should not eagerly condemn everything that is "sexually explicit" and hand-wave it away ("Yuck, of course no one wants anything to do with that.")

On a site called Hacker News, people should grok that, but the audience around here isn't what it once was. Do you people want Gilead? Because this is how you get Gilead.

I highly doubt a legitimate newsletter or mailing list for sexual abuse therapists would be sending out "sexually explicit" content.

It's not insanely difficult to start your own mailing list/ newsletter service. At my previous job, I had did exactly that. Then you can absorb the potential legal liabilities yourself.

If they host mailing lists, that's something I've never stumbled across.