Would it be completely disgusting if I steal my client's underwear to masturbate with it
That does sound gross. What exactly gave you the idea of doing that?
I've got good feelings about her and I've got a feeling that every time we'd do business, she'd wear some already-worn underwear, just for me. Oh and I'm also one of the milder-mannered members in the group. She fucked two of us and she's looking to fuck everyone else as well
It's okay to be sexually attracted to someone, but you need to keep these emotions under control.  It is wrong to steal anyone's possessions or to use anything they own for your own pleasure, unless you have their explicit permission.  It is also wrong to assume that other people are sexually interested in you, or in anyone else.
Okay
I'm glad you agree. I think it might be a good idea if you just put some distance between yourself and this client.