Pig sex
Pig sex is a fairly common but rarely discussed fetish where the fetish object is someone who the individual finds grossly unattractive, perhaps even repulsive. 

A pig sex fetish is different from the tendency for undersexed or hypersexual individuals to select unattractive partners who represent the easiest opportunity to engage in sexual intercourse. People interested in pig sex actively fetishize and pursue sexual experiences with others they subjectively feel are very ugly, it is the very ugliness which is the turn on for these individuals. Pig sex fetishists are often capable of having an active sex life with people they consider more attractive and tend to indulge their fetish every now and then.

Both heterosexual and homosexual males and females are known to indulge and a pornographic sub-genre has a presence on the web, usually prefixed as 'nasty' or 'ugly.'

The taboo nature of pig sex is probably a combination of two factors.

1. People in friend groups tend to judge each other by the attractiveness or personality of each other's casual or longterm partners. Openly indulging in or admitting to feeling intrigued by pig sex is likely to draw explicit and/or implicit criticism in all but the most open minded of friend groups.

2. Actually admitting to a partner that you wanted to have intercourse with them because they are the target of a pig sex fetish is unlikely to be considered as a positive affirmation by them. This is the case of course for a partner who represents an object of any kind of fetish, e.g the morbidly obese, a member of another ethnicity, people with disabilities etc. However I hope most readers with a consideration of other people's self-esteem would see what was especially wrong with saying that "I wanted to have sex with you in particular because I am fascinated with how repulsive I find you."
"You what, you actually shagged that fat lorry driver with the ear hair!?!"

"Sometimes I just want to have pig sex."