It’s all about fantasy to me, so I like the aesthetic of restraints.
I like the look of ties and knots of ropes, and the patterns one can bind, and the imprint it can leave on skin.
Or the cold steel foreboding look of chains and manacles on soft wrists and ankles.
There’s also the enticing sounds of metal on metal, like cuffs on a metal bedframe, that just accentuate any thrust sounds of skin on skin, or the squeaking of a mattress upon bedspring.