Theology majors may represent a greater proportion of believers in pre-marital sexual abstinance than do business majors, therefore weighting the odds in favor of marriage.
I don't believe you need a massive set of data to reason that people who study theology are more likely to be religiously inclined though. So I'd say grandparents theory appears more likely.
Some of the context missing is many theology students intend to go into the ministry -- becoming a pastor. Or a missionary. Many of the people on this track are intensely passionate about this calling.
Many couples pair up because their goals coincide and they decide to do this together as life long partners.
Or, to put even more simply, the intersection of their shared values is much much higher than business majors.
Or maybe Theology majors have a higher pregnancy rate and "need" to marry more often. ;)
Any theory is as good as any other without more data or a way to test it.