wahoooo!
Canonically Mumble would be asexual I think, doesn't find others attractive and has a generally low libido. Their spikes also come up defensively when they are uncomfortable with being touched, but with enough subduing...

I've also yet to mention but the core of Mumble as a character is that he's almost always incredibly anxious.
He is always in some form of anguish, but he's not quite a masochist in the sexual sense.
He's complacent with the abuse he endures. More than anything, he becomes suspicious and on-edge when things are calm and quiet... ironically it causes him more fear and pain than the constant abuse he's used to.
So even though he's terrified of Screech, he clings to her, abuse is a familiar comfort.

All of the characters I draw are mature + intelligent, unless stated otherwise (with the occasional exception of brain-damage...)