There are few people in the world who have not heard about guppies and their rather sordid desire to breed. People the world over associate the fish with someone who has less then perfect morals and this is a really bad rap for the fish themselves. It is nothing more than natural instinct that drives their constant breeding and mating cycle.
There is the matter of being able to house the guppies that you own if they continue to breed, which they will do until they die. A guppies population can get out of control in a hurry if you are not careful. This is the main reason why people have avoided guppies as a fish over the years.
In the wild the guppies breed for survival. In their native land they are a main source of food for other larger fish. Their constant breeding is part of the natural circle of life for the area and helps to maintain their population as well as that of the local fish that eat them. So as you can see there is nothing out of the ordinary about their behavior. Actually they are quite in line with the rest of the fish population. Some fish like guppies breed often and have small amounts of eggs each time. Others breed once a year and have thousands of young at once.
The problem is that people do not always have the time to take care of a rapidly growing family of guppies so they avoid them. There are two methods to help that seem to work well.
One, you could only buy one guppy. This is the best idea if you happen to like the fish but do not want any kind of family starting in your tank. Do not let any fish store owner tell you that you are getting two males or two females. There is no way of knowing outside of lab tests that the owner cannot run.
The other method is to avoid buying guppies. This has however gotten a bit trickier. In an effort to sell more of the fish some less then moral pet store owners have taken to naming the fish using their scientific name rather then guppies. This means that the person buying them does not know what they are getting.
If you are not in the market for guppies then you should ask or look up the name of the fish that the person is trying to sell you to avoid any kind of problems.