How can Jewish Greg marry gentile Pam without being excommunicated?
I mean, Greg is clearly a practicing Jew (he never says he isn't).
And Jack and his family are anything but Jewish, and being so conservative, must be religious (baptist, presbytarian, something like that).
So unless Greg is a Messianic Jew, how can he marry a non Jew without being excommunicated? Only way is if either one converts, and the movie just never goes there.
Never mind how the kids will be brought up (as Jews or as Christians), something Jack would definitely want to know, after seeing his reaction to the mere suggestion he may have some Jewish ancestry...