The term "bent over" is used when being physically punished, or for a position used to gain entry to the vagina or anus for sex. Also when a person is being used, treated badly or taken advantage of.
When I was young and very naughty I was bent over my father's knee and spanked.
If ever my boyfriend comes home at lunchtime, I know I'll be bent over the kitchen table and shagged.
One is considered "bent over" when they are being played, used, or stepped on. It a metaphor between when someone is used for anal sex where the person performing the sex feels extra pleasure do to the tighter orafice, yet the person receiving the sex feels nothing from it, and when someone is taking advantage of someone else for a personal gain that doesn't help the other side.
2. This kid called me a faggot, and before he could say anything else I power bombed him to the ground and stabbed him in the back with a knife, man was he ever bent over!