“Why do Christians pray about whether to marry someone, when Paul says that if they want to get married they should just do it (1 Corinthians 7)?”

I have to assume that this question has to do with whether or not the Christian should marry a particular person, rather than whether or not to get married in general. Let’s assume this reading, and look at the text in question:

But I say to the unmarried and to widows that it is good for them if they remain even as I. But if they do not have self-control, let them marry; for it is better to marry than to burn with passion.
- 1 Corinthians 7:8-9 (NASB)

This text is not a “just do it” text, not by any stretch of the imagination. This text states that it is good to be unmarried, but that it is perfectly acceptable to get married. It does not, however, state that you should just rush out and get married.

If for no other reason than one of practicality, you should probably attempt to find a good marriage partner. Some (or much) prayer will most likely be involved there, as it should be in all important decisions. Once you find that partner, however, go ahead and get married. It’s that simple.

If the question is actually about whether or not to marry or remain celibate (as those are the two Biblical options), prayer should definitely be involved there as well. If you have desires that lead you toward marriage, but for some reason feel you’re called to remain unmarried, either you need to get married or you need to have peace with being celibate. Either way, it should be a matter of prayer.