Gay pig sex is someone who is into extreme limits of sexual activity, in that they push the boundaries of what is, what isn't, acceptable.
There are actually two distinct meaning for the term.
(1.) A person who is eager for any type of sexual encounter, irrespective of most 'standard' limits. This can involve all sorts of sex, such as being into hard core fisting, into urine, and other 'extreme' fetishes, that have a sexual connotation.
It does not mean simply bareback sex, but the more edgier type of sex, and is generally used in the Bear or BDSM community as an alternative for 'nasty sex'.
Simply put, a 'sex pig' is someone who will try anything, at least once. They are someone who seeks something new, no matter how great the risk might be, but they do set their own limits.
It is just that those limits appear to the casual observer as being inconsequential.
(2.) A person who engages into sex with another, who is attracted to that person, for their lack of beauty or physical appearances.
Similar in nature to a 'pity fuck' or to 'dog fuck' which is generally associated with College Fraternity Hazing. That being where pledges are sent out to find the least attractive person, have sex with them, or in the more 'standard' format, to bring them to a dance or social gathering.
In some instances, it can be the person's unattractiveness that draws the other, that arouses them sexually which more 'good looking types' don't do.
Generally this refers to where the idea of having sex with a person, who is considered repulsive. This can be due to many different physical traits, such as having acne, to being obese, and everything in between.
Another form of being a 'sex pig' is of one attracted to people who are obese, or considered to be overweight.
In some communities, the term can also simply refer to someone who is a sex addict, who seeks sex constantly.