in that case then the term "self-preservation" is evident on their belief that it's illegal to marry someone outside their faith... even though we're both christians (they don't consider we catholics as christians though, they have their own reason and explanation, but their stand is not like
Pein's, if you're familiar with him)...
following your statement based on my understanding, when outsiders marry one of them, the outsider, converted or not, might tend to influence the resident one to change course or weaken her faith... then in that way i can agree on your term "self-preservation"...
but i can't affirm that they only focused their faith among themselves, they conduct weekly(?) tasks wandering around and spreading their word (it's a must, and their preaching hours is recorded and will be reported, individual).. but in a passive way not like those noisy sects in the sidewalks using megaphones and etc... IMO they need not worry upon themselves anymore because they're confident that everybody on their grounds have strong faith.. as what she manifested... if one of them astrays, then it's understandable that he/she abandoned their faith, and will be disfellowed... they all know that... so as of now they're spreading themselves fishing out those individuals who have weak, troubled faith...
so aside the given verse, is there any other biblical passages that says not to marry other individual outside their doctrine?
and bdway, i ain't a god of wealth or anything... am not even rich... she just pointed me out this verse...