I've looked over the man page for `ssh' and `ssh_config' on several systems and I think people have it backwards. The pages I read indicate that `-X' is more secure than `-Y'. Under the `-X' part on several systems I have access to says:
"For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by default."
The `-Y' says:
"Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls."
I would think being subjected to security controls would be more secure than not being subjected to them.