In order for that grandfathering clause to have any significance whatsoever, ICM Registry has to win approval to be the registry for .porn, .sex and .adult, first.
There's no way that's going to happen before the first renewal period for .XXX domains. Each gTLD application has to go through a rather long approval process, then all applications are subject to objections by various groups with standing to object (and believe me, there will be many objections to .porn, .sex and .adult), and there's no guarantee at all that ICM will ever win approval to administer those three gTLDs.
In other words... probably wiser to not count your chickens before the egg has been laid, much less hatched. ;-)
I outlined all of this in
a post last year -- but you needn't take my word for it on any of this; you can read
ICANN's gTLD Applicant Guidebook for yourself. Sure, it's long and dry, but if you are staking your assessment of your .XXX domains' value in part on the prospect of eventually owning the .adult, .sex and .porn versions of those same names, you owe it to yourself to understand the process and the issues.