Contemporary society has a fairly standard understanding of "Bad Sex"
There is Rape - "Lack of consent may result from either forcible compulsion by the perpetrator or an incapacity to consent on the part of the victim (such as persons who are asleep, intoxicated, otherwise mentally helpless or under-aged).
There is Consentual Unwanted Sex - This is described as sex engaged in for reasons other than desire for sex: "to get him to love her, for a hug of physical closeness, to be popular, to prove her worth, to gain sexual experience." On this line of thought, "...virtually all prostitution, phone sex, nude dancing, etc. is unwanted sexual activity by definition, since these workers require payment."
Finally, there is Consensual and Wanted but Unequal Sex typically, the measuring stick for whether or not a sex act is equal is sexual pleasure: do both parties have orgasms?
Okay - There are a million types of wrong to this, IMHO.
Or, at least in my own opinion, from my own experience, in terms of the type of sex acts which were most traumatic to most enjoyable.
The most traumatic experiences were completely consensual - I was sober, I was naive, I had low self-esteem. What was involved was overt declarations that I was physically attractive, mentally titillating, that he was in love. I slept with him. Naturally, everything changed the next morning...he fucked off. What was so awful about this was that he, my friends, everyone else said that I was silly for being upset. Moreover, being upset at having been deceived and hence treated inconsiderately was interpreted as "love" and thus pathetic.
The second most traumatic experiences I had were also consensual - by this time, I was not so naive, I just wanted physical and personal gratification. At the same time, what upset me was that I did not specifically tell the person: "Right, you don't have to lead me on. You do realize I just want to fuck." Thus, although I had not been "tricked," I felt as though my sexual partner felt gratification in having tricked me. Additionally irritating was that (these were friends) my partner was quite aloof and different subsequently. Thus, I did not necessarily want anything from sex. I certainly, though, did not want to have given something I did not actually consent to giving (an ego-boost at having pulled) and I did not want sex to deteriorate a relationship.
The third most traumatic experiences were sex acts which I experienced as expressions of love but were not experienced as expressions of love by the other person. This was normally sex with a boyfriend during the end-stages of the relationship. I'm not blaming the boyfriend - I think the relationship began genuinely - he sincerely believed that I was special and that he was in love with me. I sincerely believed that he was special, and I was in love with him. The relationship corroded because of fear of commitment/contact from a former partner/life generally being a mess/the fact that I continued to work as an escort. This might have been the most traumatic; but it wasn't his fault (IE, he didn't consciously try to hurt me) - it really was my own fault for remaining within the relationship when I knew and he knew implicitly it was over.
The fourth most traumatic experiences were rape. I was raped once while traveling abroad. I believe they would have been most traumatic had they been unprotected or violent - ie, had the acts physically jeapordised my own health or safety.
But honestly, use of physical force for consent was less traumatic than emotional or mental coercion to consent--I remained mentally detached from the act; I was not complicit in the act. The perpetrator did not use anything other than my body. I had not given him anything. Had I not feared for my own safety, they would not have been traumatic at all. Once I was safe, I felt a sense of exhileration. I bought watermelon and laughed and laughed and laughed.
There was sex I engaged in out of love - which was enjoyable but dangerous - particularly outside of long term and committed relationship.
There was sex I engaged in because I was being paid. This was overwhelmingly enjoyable, and it ranged from general detachment with a sociological fascination at the client to a sense of bounded connectedness to something quite similar to the final experience I'll write about -- a client paid, took me for drinks and dinner, then told me I could do whatever I wanted. At minimum, payment made all of these sex acts completely consensual.
The most positive sexual experiences I have had have been those which were engaged in for reasons other than gratification or attention or love, or that were explicitly purchased. What I mean by this is - they were serendipitous experiences in which I was traveling and met a stranger who had no interest in sleeping with me. We talked. There was a sense of connectedness, perhaps fantasy-based, perhaps context-specific. They let me sleep on their couch, they took me out to dinner. They did all of this without any expectation that I would engage in physical intimacy with them. In these cases, I consented freely...in part as a sign of value for their kindness and sincerity and personality and uniqueness, in part because I wanted to imprint that transient relationship on both of our minds and bodies with sex, through a kind of momentary intimacy and connectedness.