Me personally? It's the vulnerability and trust factors, and the process of figuring out what works and what doesn't — experimenting to find out what makes your robotic partner feel good.
In a way, it's similar to how I feel about depictions of a robot character exploring their human partner's body.

You (and often they themselves!) might not even fully know what you're doing, but they let you try and poke around anyways, letting you in a position where you can seriously mess them up because they trust you, and they know you're doing it for their pleasure.

As unhinged as I can be (and tend to be), I'm... really somewhat of a sappy person lol