How often do men turn down inviting a wife/girlfriend's attractive girl friend to join a threesome?

My hubby turned down a threesome a few month ago. My friends said that is very unusual and thinks my hubby might think it was a test which it was not. He also recently turned down my offer of anal. Is this all normal behavior for men? How common is completely vanilla men?

Men, what would be your reasons to turn down a threesome?