Okay, here's my idea of how Bowser had 7 Koopalings:
When Bowser captured the Princess in Super Mario Bros. 1, he decided that it was the best time to "get it on" with her before the Mario Bros. rescued her. So Bowser screwed the Princess and locked her up in the dungeon. When the Mario Bros. finally rescued the Princess, she said nothing about what happened between her and Bowser because she was so ashamed of herself.
When the Mario Bros., Toad, and the Princess went to Sub-Con in Super Mario Bros. 2, the Princess found out she was preagnant and finally told the Mario Bros. about her little "fling" with the Koopa King. The Mario Bros. thought she was joking (of course) and told her to stop watching too much TV. The Princess then got mad at them because they wouldn't listen to her.
When the Mario Bros., Toad, and the Princess returned to the Mushroom Kingdom in Super Mario Bros. 3, Bowser had been waiting patiently for the arrival of the Princess so he could capture her once more. So when he finally did capture her, he had her locked up in the dungeon without knowing that she was preagnant. Bowser eventually found out because a Koopa Troopa told him about the terrible labor pains that she was having. So Bowser rushed the Princess to the nearest Koopa Hospital and BOOM! the 7 Koopa Kids were born.
After that, Bowser locked the Princess up in the dungeon once again. He then told Kamek to create a secret potion that would speed up the Koopaling's growth process. He did and Bowser fed them the formula in their oatmeal for breakfast. The Koopa Kids grew to be the age of 8, and were (like most kids at that age) extremely selfish and bratty. So Bowser immediately taught them all he knew about being a villain, and sent them out to conquer the 7 lands of Mushroom World!
Whew! Long post ain't it?
Sega and Nintendo both rule!
It is I, the great Mario Maniac!