In human relationships such as truth-telling, not harming others, and sexual relations, are there some activities that are 'natural' to humans and others that are unnatural? If so, why or why not? ( in regards to natural law theory)

Yes , there are some relationships between human beings that are natural and also others which unnatural .  Natural law is a theory in ethics and philosophy that says that human beings possess intrinsic values that govern our reason and behavior, humans have a natural drive to do things for instance humans eat, drink, sleep and procreate. 

Step-by-step explanation

 Yes , there are some relationships between human beings that are natural and also others which unnatural .  Natural law is a theory in ethics and philosophy that says that human beings possess intrinsic values that govern our reason and behavior, humans have a natural drive to do things for instance humans eat, drink, sleep and procreate. These actions are in accordance to the natural laws for species which enables the species to survive and procreate the natural laws provide a guidance in humans a relationship through the law of survival the natural action of living things to maintain themselves and reproduce.  However, some activities are not determined by the natural laws for instance the law of not harming others may change in accordance with the consequences and the context. for instance abortion is wrong according to natural law as it breaks the primary concept to reproduce. In the case where the mother life is at danger the abortion might be carried hence this activity is unnatural as per natural law but it is also for the good of the mother. The relationships of people might be reliant upon the social class , culture education and upbringing thus in some cases that activities are determined by the unnatural laws and that some activities depend on the context nd the consequences they might attract