This is the deal on marrying outside the organization. As of the 11/1/1989 Puketower, you cannot get disfellowshipped for marrying outside the congregation. However, you can lose certain privileges for a time. You cannot use the Kingdumb Hell for said marriages, and the hounders will not solemnize it (whatever that means). And you can be "marked" as walking disorderly.
That could change in the future. They could be working on making it a judicial offense, or not recognizing the marriage at all (hence anyone that marries outside the witless congregations would be viewed as unmarried and hence committing fornication). They could even make it a judicial offense and not recognize it if the potential mate has not been baptized long enough (again, whatever that means--usually a means of taking it out from under someone's feet that they had the Value Destroyer Training School in mind for). That could come out in a future Kool-Aid Puketower or a Kingdumb Misery down the road.
Why? If someone marries a non-witless, that person could easily do things to undermine the witless's faith. The unbelieving mate could, for instance, hook up a computer and browse apostate sites without the hounders being able to do a damn thing about it. The non-believer could also point out scriptures taken out of context. The non-believer might go to the Kingdumb Hell one day and take notes of every time the speaker takes a scripture out of context or uses specious or fallacious logic. When this gets discussed, the children and/or the witless spouse could be knocked right out of the illusion. And, the non-witless is more likely to see the pedophile problems and other scandals within the Watchtower Society and do something about it (like putting up extra Christmas lights in the yard).
And, they want another way for all the "brothers" they can corral into the Value Destroyer Training School. This means they have to stay single as long as possible for them to be eligible. Of course, single "sisters" are worth less to them, but they can pioneer longer as their biological clock runs out and they grow old and destitute.
I see no legitimate reason why two people of different religions cannot marry. This is no different than different age groups or cultures marrying. It can create areas of disagreement, but not insurmountable ones. Only when one or both are so stubborn that they will not look at the other's viewpoints can marriage outside one's religion be looked at as causing serious problems.