In essence, a 'snugglebum' is a human heater. It is the guy/girl that you can cuddle with on a cold/rainy day without any sexual tension between you. It is when two people (who are madly in love) snuggle together all day instead of getting up and doing things that need to be done, (like chores, errands, homework, even socializing with friends, cleaning, etc). It can be used as a term of endearment as well and although this may seem to be emasculating, it is not. The boyfriend (or soon to be boyfriend) will usually want to go do something else and he may even get bored with this mundane task of cuddliness but if he loves his girlfriend enough he will lay down with her and be called a 'snugglebum'.
Michael: "It's raining out, what do you want to do?"
Amy: "I'm not sure, it's kinda cold..."
Michael: "Do you want to lie around cuddling all day?"
Amy: "Yup, let's watch a movie and snuggle!"
Michael: "fine....we'll be snugglebums"