Here's a few reasons to start off with:
1. They are both edible.
2. It will make Marvin miserable but make the rest of us happy.
3. Turnips bring out Marvin's evil side.
4. They both like drowning kittens and lighting preschoolers on fire.
5. Marvin's already married to Aeva, so by marrying my turnip he helps preserve the age-old Random tradition of polygamy.