I still don't know what code you're using, .
The book I am reading is a general construction book it covers everything from grading to roofing. The book dose not tell me specifically what size vent I will need for a toilet; however, it dose have tables soused from: "UBC Illustrated training manual (Walnut, calif: International association of plumbing and mechanical Officials 1985)."
In one table I read that a private toilet counts for 4 fixture units. The next table shows 1.5" pipe being capable of venting 8 fixture units. Based on these two tables it appeared that 1.5" pipe could actually vent 2 toilets.
In the houses I have worked on I always see 2" vent pipes for toilets, add to that your reaction and the fact that I don’t want to get into a challenge with my inspector, I am definitely not going to install a 1.5†vent on my toilet.