Hi Amy,
Not a stupid question at all. A lot of people ask that question and a lot of other people often give the wrong answer so it good to have it come up once in awhile.
You will not get mixed fruit. Your plants will produce what they should to name. This is of course if you grew the correct plants to begin with. I think this where some people go wrong in their ideas. They see their plants are growing incorrect fruit and believe their plants have crossed when in fact they never had the right plant to begin with.
You could possibly get cross-pollination which would affect the next generation of plants from seeds you save. The chances of that are low. The crossing rate of tomatoes isn't very much. The flowers are self pollination, and the flowers are not very attractive to bees compared to other plant's flowers. Of course if they are the only thing blooming, you will probably have more insect contact with those blooms. But even if crossing occurs, it will only affect some of the offspring.
If you have the time and inclination, you can bag the blossoms to ensure seed purity. If you cannot bag the blossoms, collect as many fruit as you can to take seed from, because as I said only a small percent of the seed would be crossed if any at all, but at least you know you will have many true to type offspring.
Hope this helps,
Remy

Remy,

Wow. Thanks so much for the thorough explanation. I understand it perfectly now! I don't know why, but so often I think I understand the ins and outs of some gardening question and then start to think about it and -voila! It's all disappeared somewhere within the nether regions of the old noggin.

I do think, though, that actually doing these things is what cements it in the mind, and this is the first year I've actually ordered, planted and had heirloom tomatoes grow. Usually, I just look at them in the catalog or they don't get past seedling stage. Hopefully, now I have a good start on really doing some heirloom gardening!

I'm so looking forward to the Seed to Seed book and finding out more about how to really effectively seed save. Too bad I don't have more energy to plant all the things I'm interested in. My "imaginary" garden is about 5 acres, whereas my "real" garden is only a few rows. Ah well.

Thanks again for the very helpful information.

Amy