It appears to me that you assumed he only does multiple romantic relationships without any indication to that effect. As you've just recently become involved with him, why would you expect to know everything about him by now? If you've never asked about whether or not he also does casual sex, why would you assume one way or the other?
No, I didn't assume anything. We have talked about the relationship/sexual aspects at length. He said he doesn't do casual sex. And he said he discussed potential new partners with his current partners before engaging in any kind of relationship.
So it was jaring to see his post. If that is what he wants, it seems he has a responsibility to tell me that he is seeking/having casual sex otherwise I don't really know what I'm agreeing to. (especially if he did set up the start of the relationship that he wouldn't be having sex with anybody he wasn't involved with.)