Why doesn't Victor want to marry Elizabeth immediately?

Victor is initially hesitant to marry Elizabeth. So much so that Elizabeth thinks that there is another woman involved. But Victor, as we might expect, has other things on his mind—in particular, the monster. During the meeting with his hideous creation on the glacier, Victor reluctantly promises to make the monster a bride of its own. The monster will then take her to the jungles of South America, well away from prying human eyes. The monster persuades Frankenstein to do this by telling him that having a mate will mean that he will no longer have to kill people. Victor, possibly scared at what the monster might do to him, refuses to marry Elizabeth until he has cobbled together a new mate for his fiendish creation.
But straight away, Victor starts having second thoughts about his latest science project. He starts to think through the consequences of what might happen if he actually does create a mate for his monster. Perhaps they would breed a "race of devils" that would unleash terror upon the world. One terrifying monster out of control is bad enough, but a whole race of them? The very idea is just too horrible to contemplate. So Victor destroys his latest project, infuriating the monster peeping at him through the window. Frankenstein's monster immediately vows revenge, warning Frankenstein that he will be with him on his wedding night. Victor now has an even greater incentive not to rush into marrying Elizabeth.