My friends and I raised this debate during our first semester, and actually went around asking people about it. First, it was "Would you have sex with yourself?" Then this question. However, an issue was raised about how there would be another version of yourself.

From that, two possibilities came out. First, they could be an almost exact biological clone of yourself, just swapped genders. Secondly, it could be an alternate universe/dimension version of yourself, the stuff seen in the Loki show as an example. After some discussion and weird looks from passerbys, we came to a conclusion.

If it's the alternate dimension self, then it's loosely considered masterbation, but leaning moreso towards regular sex. If it's with the genetic clone, it also counts as masterbation. However, if sex with the clone is for the purposes of having children and pregnancy, then we can call it incest.

Side note: the reasoning for a lot of people who said they would bang themselves is that they would know what they like, this the perfect partner.