It is STRONGLY SUGGESTED that JWs not date or marry outside their religion. It is not a disfellowshipping offense, that is the elders wont convene a judicial committee and boot the person out ordering all members, including family to shun them.
But JWs marry outside their religion; perhaps about 2 every year per congregation. If the JW is a man, he will lose any "privileges" or special stature in the congregation for at least a year. If the JW is a woman (women have fewer privileges), she will not be able to be a regular/auxiliary pioneer (need elder approval). Both will probably be marked, which means they will be socially shunned by JWs for an indefinite time period (JWs know when that is up when they see the elders socializing with the couple).
I married outside my religion and this is why I know from experience not theory. Eventually my spouse did become a JW, but a black cloud hung over me for the whole time I was a member of the Watchtower organization. We are both out now and still happily married and glad to be out from under the thumb of the WT organization.