I hope I put this in the right forum. If not, I apologize!
I'm fumbling around with a storyline that I'm not sure about the exact category it would fall into.
The story would involve a pair, person A and B are in a relationship, but then person A has an affair with person C. Person A does not have relations with both B and C at the same time. All sexual activity is one-on-one.
Would it fall into a pair, with the pairing being the original relationship, or would it be a threesome because one character is having relations with multiple people?
I really hope that makes sense! Thank you in advance!