I've never heard of a casual guild stating you can't be a member of another guild ever. Hell, I know that when playing on my main, if I didn’t want the trouble/hassle or even just some quiet time I would jump onto an alt that wasn't guilded. I've also had chars that were PvE based and chars in entirely different guilds that rolled in PVP exclusively. 2 different focuses, 2 different guilds.
Seems really bizarre that this game embraces that aspect/social side and you have an apparent social/casual guild that punishes players for using this clever innovation. I’m not a member of Immortals nor would I want to be as I find the behaviour of the officers and some members that frequent here to be childish on occasion. But it seems to be they are shooting themselves in the foot with this attitude towards members and the multiple guilds.
Edit - Apolgies posted after the deletion of posts.