It is natural for girls to like older guys [or guys their own age -- face it, we like guys! :-)], particularly since girls tend to mature faster than boys, and there is an allure or mystique to the "older man," particularly since they tend to have outgrown that horrid teenage awkwardness and gross sense of humor.
In my opinion, they can talk at church, if you're all there as a group, just as you would talk to your pastor or your pastor's wife, or any of your other friends or acquaintances at church. Anything else - nope! They're too young. As they get older, the age difference will disappear (my husband is 8 years older than I, and that doesn't mean much at our age, but I still tease him that he "robbed the cradle," because when he was graduating high school, I was 10 y/o), but right now, there are a lot of ways this can go wrong, and only a few ways it can go right.
At best, these guys like your daughters [assuming they're your daughters; maybe your friend's daughters] for the best reasons and out of pure motives -- maybe all the women they know their own age are world-hardened, and just as your daughters like older guys because guys their own age are "stupid", these guys have problems with gals their own age.
At worst, these guys like your daughters because they're young and naive, and let the guys get away with everything, and don't realize that the guys are actually cheating on them and only pretending respect so they can get in their pants.
Do you think your daughters are ready to get married? If not, then why are they dating anyway? Isn't the purpose of dating to discover whether the man they are interested in, is actually worth marrying?
So, again, I say in my opinion, it's one thing to talk to them at church in a group setting, but not for them to date nor to talk on the phone. It's just too much temptation and pressure for both the guys and the girls. Even if right now it's completely platonic, at some point one or both of them are going to become romantically interested in the other, and then it could become very bad very fast.
There is a tremendous potential for your daughter to be very badly hurt -- primarily emotionally, but also potentially sexually, and even physically (like, what if she gets pregnant -- labor and birth tend to hurt, right?); and only a small possibility of her emerging unscathed. Yes, it's possible that one of these guys is her future husband, and he's willing to wait for her to be old enough to get married, and they will have a lovely story when they're 80 about how they fell in love when she was just 14 and he was 20 and remained faithful ever since. It's possible, but not likely. And if the age difference is actually larger, the possibility of such a fairy tale ending becomes smaller.
I think that you should tell them that they can be friends and talk at church, but they should talk very little (or not at all) on the phone, and not go out, and not date. Tell them that you understand that they are just friends, but that you have seen a lot of "just friends" relationships go bad. That the age difference between them (13 years, perhaps?) is a huge problem, and that your daughter is not mature enough to be either his friend or his girlfriend. And that if he is the nice young man he appears to be, he will respect your parental authority, and back off. Tell them that when your daughter is older, such a difference in age won't mean as much, and you will have no problem with such a relationship when she's 18, but at the age of 14, it's just not a healthy situation for either of them. That you're glad your daughter likes being treated with respect and hope she always chooses as friends boys and men who treat her well, so this isn't anything personal against him... just against the situation.
See if you can get your daughter to read "What Women Never Hear" for more insight -- http://wwnh.wordpress.com