My understanding is that 'natural sex' is for the purpose of creating a new life, so a normal sexual relationship between partners of the same sex is unnatural sex only in the sense it does not produce a new life. Even a husband and wife having sex for love and pleasure is considered unnatural in this sense. Thus, there is nothing wrong with unnatural sex, although my personal opinion is that it should be between two people who love each other and are committed to one another.