The relationship between Mario and Daisy is strictly business. Here's the story:
When Daisy got kidnapped, her kingdom summoned the one-plumber princess-saving army, Mario. However, he wasn't inclined to as he was booked for the next decade booking deals, autograph meetings, publicity events, oh and kicking Bowser's tail for the one-billionth time. The townsfolk begged their savior to help. The king himself graveled at his feet as he were God himself and offered his daughter's hand in marriage. Imagine the bad press if it were leaked he was "two-timing" Peach with Daisy, how his career will spiral in the toilet of despair, how many sexless nights he'll endure as he plays with himself. Mario opted to help bring Daisy under the incentive they build him a castle in his honor and add in a submarine and a plane, all upfront, and in exchange, pushed them in between saving Peach from Dark Land and Dinosaur World. How else do you think he got that castle in the Mario Land 2?
That, as they say, is that.
As for Peach, I doubt she cares cause how many princesses can say their boyfriend has a castle with the world's biggest jacoozy built in? (chicks love jacoozies