To a point, once you start to talk for a while there is no reason to excuse them sleeping with others, there is an unspoken promise there. The possibility that you'll start dating. I'd even argue that if you're at the point that you want to date someone you wont WANT to sleep with others. I feel like that's just how both social interaction and the emotional dating process work. You're free to mess around until you go out of your way to talk a lot with an individual because then they get expectations. He was spending a lot of time and having sex with Fumi, but this case might be a special one because Fumi wasn't interested at first, it was just good sex.
Personally, I don't agree that talking a lot is enough to assume a relationship (even having sex isn't necessarily enough). I can understand some people having expectations from that and potentially getting hurt, which is sad - and if that happens, of course there's no reason for those people to excuse it or put up with it. But talking or flirting a lot still isn't enough to imply any kind of promise. As soon as terms like "dating," "relationship," "boyfriend/girlfriend," partner," "lover" etc. are used I do absolutely think that carries a reasonable expectation of exclusivity. But not until then.
As you say, I think this case is special anyway, since Fumi didn't even want to be in a relationship with Zono - until now? - and they haven't even discussed that. I'm expecting the plot to be the two of them developing real/deeper feelings for each other. And I am hoping it's not going to be the cliche "uke gets hurt (and jealous), and only then realises he loves the seme" scenario. But we'll see, I guess. They're interesting and a bit unconventional together so I'm looking forward to it, I hope it's good.
I really don't mind Zono sleeping with other people/person. They're not in a relationship, it's purely physical for both of them at this stage. Cheating is bad of course, but if there's not a mutually agreed-upon monogamous relationship, people are free and entitled to sleep with whomever they want. Even Zono saying "I like you" doesn't change that, people can "like" someone or more than one someone in different ways. And Fumi doesn't "like" Zono at all (yet) but enjoys having sex with him, so fair's fair! By the way, in my philosophy this is fine whether it's women, men, seme, uke, seke, whatever.
I agree with what someone said about it being an unrealistic start to their physical relationship (seriously, whut?) but aside from that this is a type of storyline I like - purely sex (enjoyed by both parties), developing into feelings.