1. The guy sprang the whole triad thing on her after LYING to her that he was broken up with his ex.
2. He is desperate and needy (not a bad thing, but definitely unattractive)
3. He's basically giving her an ultimatum here
4. He's only interested in his OWN desires.
The guy basically deserves to be ridiculed for it. Like the topic creator herself said, she's supportive of polyamory, but when the guy plans a monogamous relationship with a girl and tells her he's left his ex, then comes up with this then he's doing it wrong. Most of the people in that topic aren't laughing at polyamory, they're laughing at how badly this guy has gone about it.