I see there's 2 ways how this can go.
1. Bowser as a giant has an enormous manthing and if he were to sex Peach up, she should be dead by now.
2. Bowser as a turtle has only a very small penis unsuitable for mating. That's why all his kids are artificial.
My answer to why all these kidnappings?
Bowser wants to eat!
That's why he invades mushroomkingdom. He stores the best of the mushrooms he finds in back up castles. That's why Mario always finds mushrooms first.
He then takes the princess into a furnace like room and like in Hansel and Gretel tries to get the peach fatter and fatter till she's right to eat. Like the story goes, Mario comes to face Bowser and save the princess. Doing so he pushes Bowser in the stove to fry, just as Gretel did in the tale.
The tale of Mario is thus nothing but an inventive version of Hansel and Gretel.