I've had "contaminated" females produce 2 or 3 drops of fry before having batches that are consistent with the males present.

Some of the best "contaminated" fish I've had come from aquariumfish.net.  I've got some supposed "purple dragon" fish that came from their facility already pregnant.  I kept them without males to see what I got.  Wow!  I have two tanks of really colourful fish from solo mothers of a supposedly fixed strain.  Everything from reds, to blues, to purple, the occasional white (not albino!), silver and gold skinned fish.

A longer answer:     
Bred properly might mean that you matched up your finest male with your best female in terms of size, color and finage. ;) That way your strain doesn't degenerate and may even become more impressive. I'm really pleased that this seems to be a goal of yours.

Guppies, for their part are good at "effective breeding" which, despite the awkward grammar, is to say, they usually get the job done, whether we wanted them to or not - but maybe not when we think. Those males, of yours, were indeed just practicing unless the females just dropped.

In the Pregnancy section of the Immediate Help section (see the upper right hand corner of every Guppylog page) one can find a couple of comments along the line of ...

"... If there is a male you really want to mate her with, leave her with that male ONLY and he will inseminate her in the two days after birth. More of the fry will be his next time around. The third batch, if he is her only consort throughout that period, then almost all fry should be his.

You may not want the earlier fry. Feed her and any tank mates well, (See the references on fry and feeding in the Quick Links) even if you don't want those offspring. Guppies, usually, can be conditioned to leave their fry alone. If they are starving when fry are dropped, they will naturally eat new-borns. Then we are conditioning them to be cannibals."

Females, as if the males needed any more encouragement, secrete a pheromone that alerts males to the possibility of actually fathering fry. Males will be seen really fussing around very full females and I wonder if they are already leaking that pheromone, sort of like the water breaking with humans. That certainly introduces the guppy males to that crucial 48 hours.

Lab studies, perhaps field studies, suggest that females exercise some choice among males. In optimum conditions they will look for the larger, more colorful, most symmetrical (balanced in features and color) male. If he is the best dancer, going through the sigmoid display best (thanks PeterW for finding that), so much the better.

Females, given a choice of males, will select up to three males. There seems to be an increased survival potential in genetic diversity - in the wild. (That is just the opposite of what we are doing when we "line breed" guppies.) Unwelcome males may be chased away, attacked and if, really persistent, may be bitten and even have a portion of his gonopodium severed.

Despite all of that craziness, there are still small males who are "sneaker males." Without fanfare they will try a surprise "jab" at the female. Percentage of success is very remote per try. But we see enough undersized offspring that we really don't want those smaller males in with breeding stock. (That is why serious selective breeders will be so quick to separate out females from the other fry when females show their gravid spot at about one month.)

So when the female(s) of your choice is/are near to dropping fry, place your male(s) of choice with her/them. Those initial fry may be given away if they were not from the pair of choice.

Guppy breeders of fish intended for show keep detailed records of who was bred with whom and which offspring were bred and which fry were saved. This could be on a clipboard, studbook or database.

Hope this helps. Glad to see your interest in raising specific guppies.

When you have an evening, click on the Immediate Help Section of Guppylog and read over those first five or six sections. Some of the info should be very useful. :)