I met this wonderful girl sometime back and we had a great thing going. We understood each other very well and have lived in great pleasure and happiness. Initially we started dating. after sometime dating, for unavoidable circumstances we knew that this relationship is not gonna work out long-term.
So we had a friends-with-benefits relationship just to have sexual satisfaction with no feelings. But with time we generated strong feelings towards each other up-to a level that we started a relationship. But the relationship is temporary. There is a expiration date. We want to do everything as a couple together. With heavy deep discussions and deep understanding we made this decision. We both are deeply in love but we are still honoring our agreement on the termination.
We aren't really bothered of finding another partner yet so that's why we are having this temporary relationship. When we feel like this is enough, we will terminate it. But no-one knows about our relationship. Not even my parents or hers or our friends. It's secret one.
Now to the important part. As a couple we are thinking of having sexual intercourse. I'm still a virgin. I love her a lot. And I do want to engage in such. But I still haven't agreed fully. My question is, is it a Sin or Akusala if I have sex with her? If so how does it become a sin? What is the Buddhist perspective on this?