I feel that the previous answer might be a bit one sided.
0) First of all, persons who are not baptised as a JW, but only raised as a JW, are free to live their lives as they see fit (there is no infant baptism among JW's). They are not considered as JW's and the society can not officially sanction any of their life style choices. The same goes for disfellowshipped persons or persons who disassociated themselves from the JW's.
So many persons who are raised as a JW, do not consider themselves JW's (although the outside world might perceive them this way) and they live with partners whom they choose 'freely' (of course one has to take into account some pressure from family, peer pressure and pressure from society in general, which limits the free choice of most persons).
1) Since many JW's are married and belong to a different religion before they get baptized as a Jehovah's Witness, interfaith marriages are not so rare among JW's (typically only the wife converts to JW's).
In those situations Paul's advice to the Corinthians apply. Basically he advises to stick with your partner (1 Cor. 7:10-16). So in that case, JW's are married with non-witnesses and have to do everything in their power to make the marriage work.
2) Persons who are unmarried adults and baptised as a JW, are encouraged to marry only other baptised JW's (of a different gender). Again this is based on the advise of Paul to the Corinthians.
- 1 Cor. 7:39: '...she is free to be married to whom she wants, only in (the) Lord'.
- 2 Cor. 6:14-15: 'Do not become unevenly yoked with unbelievers. For what fellowship do righteousness and lawlessness have? Or what sharing does light have with darkness? (...) Or what portion does a faithful person have with an unbeliever?'
Baptised male JW's who do not follow this advise, might lose some 'privileges' (e.g. they may not teach the audience in public anymore).
It is however not a disfellowshipping offence to date or marry a non-JW.
I do not know if there are any official sanctions against female JW's who marry non JW-men (the position against women is more relaxed, I think).
When they do marry, the same advice as given in 1 Cor. 7:10-16 apply.
3) Many persons who are baptised as a JW, fade away and become inactive (this means they do not go knocking on doors anymore for a period of 6 months or more).
According to some studies 2/3 of the children who are raised as a JW, do not consider themselves as a JW when they reach adulthood (this is apparently the lowest retention rate of all the religions).
Most of time, the inactive single persons are also free to do whatever they like, because the congregation of JW's will probably not keep track of what they are doing.
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,...). In that case, the same advice as given in 1 Cor. 7:10-16 applies.
Often the JW partner is quite disappointed in the decision of the partner and this can put a strain on the marriage, but in many cases the relationship continues as usual. 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. So sometimes situations might arrive where e.g. JW's give a party and only invite the JW partner and not the 'inactive' mate.
It is also not very uncommon that young persons who are raised as a JW (and baptised) marry each other and once they are married both stop practising the religion.
So in these cases you have relationships between real JW's (who go door to door and to all the meetings) and only 'name' JW's (inactive ones who do basically nothing).
The bottom line is that JW's are allowed to marry outside the faith, but it is discouraged.