This is what I also suspect. I do think a lot of them haven't the first fucking clue about the subject.
Tbh I'm surprised Tim Farron signed in the first place, however it's a difficult one for him given he was thrown out as leader over his personal views (which don't reflect his voting record) on being gay.
I suspect he's trying to work out where the public stand on this - it would explain why he initially might have signed (as he's told by fellow MPs this is the thing to do) and then had correspondence or views from gay constituents / friends explaining things in full.
Others will be supporting/ not supporting on the basis of what they think constituents want. So student heavy MPs will be more inclined to support. (Which is why it wouldn't surprise me if there were GC signatories).
The key take away is NOT who has signed it though. It's the number of MPs who have not. Remember parliament COULD in theory change the equality act to fuck over women and gay people and effectively over rule the SC ruling.
As I've suspected for a long time, there just aren't the numbers to do that. And even if there was a rainbow collation, I still think it would be difficult to pass such a change. They KNOW how unpopular and controversial doing this would be. They know the subject is utterly toxic. And that is the most important take away from this.