So yeah, I was thinking about this. Marrying an unbeliever is considered to be WRONG, but at the same time, it is NOT a disfellowshipping "offense". Why is that?
Well, the reason is simple: there is more to gain than to lose by NOT disfellowshipping. If they did disfellowship everyone who married "out of the truth", there is a high chance that such ones WON'T be coming back and the org would lose them forever. So why not take the softer approach - make it permissible, with the hope that the believing one can eventually bring their mate AND any children that they may have into "the truth!" This would be an acceptable solution for many single ones (especially sisters) who are not able to find a marriage mate within the org. So while the org considers marrying a "worldly person" to be wrong, they still allow it!