Depends on the exact fruit. Some fruit, ie squash, have male and female flowers, one of each is needed. A bagged squash blossom is good for decorative and lightly flavored stuffed dishes, and nothing else.
Peaches have male and female parts on the same flower. Self fertile flowers just need pollen from the male part to move to the female part. These are called "perfect" flowers. A single bagged self fertile variety of peach blossom can pollinate itself either by being jostled, or by the stamen growing through the anthers as it opens. In tomatoes, this often happens before the bloom even opens, and the first thing you'll see on the end of the bloom is the already fertilized pistil.
Edit- If you look in the hybridizing fruit posts, I believe there's some discussion of the need to neuter self fertile flowers if you want a cross, this is what they are talking about; cutting the male parts off the perfect flower before the female parts are receptive.
