There are different reasons why some sisters and brothers choose to marry unbelievers, either voluntarily or out because they have drifted away from close association with brothers and sisters.
It is also possible that two partners are baptised and only one of them (very often the husband) becomes inactive (and has a different outlook on life, becomes agnostic, atheistic,...). Also the relationship with other JW's can continue as usual, however some JW members might not be so cordial anymore with a person who has become inactive. JW's give a party and only invite the JW partner and not the 'inactive' mate.
The reason why others got disfellowshipped is not their marriage to a non-Witness in itself, but the immorality or pre-marital sex involved, which is very common when a Witness develops a relationship with a non-Witness, simply because they may not share the same views on this matter.
However, dating couples who are both Witnesses are not free from the dangers of immorality even though they both should have the same views on pre-marital sex.
Even people who are not Jehovah's Witnesses and belong to other churches would find it wise to marry someone who shares the same faith.
It is just one of the many compatibility aspect of finding a mate.