How does every other post get a NSFW tag but this one doesn’t?
Anyway, if you swallow a fair amount of juices, you may get a calorie but it won’t take you out of a fasted state. That being said, depending on how long you intend to fast, having your face down there probably will give you a strong urge to relieve your own sexual tension. If you plan on going more than a couple days, orgasms will drain your energy enough that it will make it harder to finish the fast.