I think it depends on the fans... Some do, like the SMAP fans, or Fukuyama Masaharu fans, and of course the Arashi fans... And I think the most recent would be the Perfume fans (most recent scandal). But I don't seem to see it as much, or at least with as much intensity, for some celebs... Maybe it's just a matter of how popular they are? Or the image of both parties? When Inocchi and Seto Asaka were rumoured to be dating, and even when they confirmed that they were dating, and eventually got married, the response always seemed to be relatively positive, because both have clean images and it was a long relationship (so it doesn't look like they're both players).

I thiiiink the vast majority of fans are okay with MatsuJun and Inoue Mao's relationship because of the Hanadan connection and Mao's good image (plus it's been freaking 10 years). If it's even true to begin with lol. Every few months they're rumoured to be getting married but nothing has happened, latest article says wedding bells will ring later this month or next month, but who knows? Ironically, if it's false and MatsuJun were to announce that he was marrying someone else, I think the fallout would be intense XD

Since you're interested in Fragile, apparently MJ was 内定-ed to be the lead, but rumour has it he had to decline it because of the impending wedding announcement, and that's how it landed with Nagase ._. idk.