If incest means "sexual contact among genetically related individuals", all human sexual relationships are technically incestuous, given that all human lineages can be traced back to an individual from whom we are all descended. That said, such a definition is not terribly helpful, but it does illustrate the difficulty in devising a bright-line distinction between "incest" and "not incest".
The reason I refer to the arbitrariness of the definition of incest is that I am pretty sure that modern incest laws were cobbled together from canon-law considerations of consanguinity and whatever religious justifications were used in Protestant domains. Therefore, they represent a prescientific approach to issues of relatedness. For instance, marriage of a "non-related" individual to one's sibling created a consanguinous relationship between one and one's inlaws, so, under canon law, it was possible to have an incestuous relationship with someone to who one was not gentically related.
I know that it is possible for people to have same-sex sexual contact with second-degree relatives (i.e., first cousins), if I am to believe the anecdotes I have heard. However, whether such contact is incestuous, depends on the legal definition of incest and the legal definition of sexual contact. In most US states, incest laws refer to "sexual intercourse" and "sexual intercourse" is defined in a sex-neutral way, so it is possible for same-sex sexual relationships to violate the incest laws. Nevertheless, I think that devising some non-legal definition of incest is rather futile because incest, as it is being discussed in this thread, is primarily concerned with the legal delimitation of sexual behavior. If one wants to discuss the biomedical implications of close relatives' procreating, such a discussion is better served by talking about the effect of inbreeding.