Huh? I'm not a Muslim (agnostic, if anything). But no religion (or any other large group of people) is a monolith. Replace 'Muslim' in your original post with 'men' or 'football fans' or 'postal workers' or 'Christians' or 'furries' and it'll be just as valid.
Some of the people who post offence on sites like this might be into the fantasy in a detached intellectual way -rather than inserting themselves or their own loved ones into what they read/see. So yes, they can absolutely take offence when someone makes it 'real'. They don't want to think about their mother (or wife or whoever) in these situations. They just conjure up a mother and wank to that.
Others might go in the opposite direction: they're role playing, everything is part of the fantasy & the role they've chosen is that of the naive/innocent/betrayed. But, if you tell someone you're just playing a role ... it takes all the fun out. So they keep going as if its real.
I also assume there are some out there who genuinely want all of their fantasies to come true in real life. On the internet, there isn't any meaningful distinction between this group and the previous.
The problems occur when someone from the 1st group gets treated as someone from the 2nd or 3rd. Or when people from the 2nd & 3rd groups want to be treated as if they're in the 1st. That creates a real mess. Personally? I don't intend on engaging with any of them. Because sooner or later I'll just end up pissing somebody off & ruining their day, and I don't want to be an asshole.