I think I'm sensing a theme here...
Any Meister/Any Meister (preferably Stein/someone, though, because he's hot. Possibly Stein/Maka? If you didn't mind it being creepy?), manipulating soul resonance for sexual purposes.
Stein/Spirit, vivisection--sharpening the blade
Stein/Medusa, fighting as a method of reaching orgasm instead of 'normal' sex.