The reveal of Jon Snow's true heritage has long been hinted at in fits and starts on Game of Thrones, but it all came together in one breathtakingly beautiful, yet uncomfortable scene in the final moments of the Season 7 finale: Jon and Daenerys having sex. But the irksome fact that Dany is Jon's aunt seems to commonly hit the defense that incest is a part of Targaryen culture. Viewers became emotionally invested in an incestuous relationship for an entire season and now have to grapple with those feelings, as weird as they are. But does the Targaryen defense really hold up?
Let's get something uncomfortable out of the way first (and it may be a reminder for die-hard GoT fans): incest is absolutely a big part of Targaryen history. Daenerys' father and mother were also brother and sister. The Targaryens have practiced incest to maintain the purity of their bloodline and because of a familial belief only true Targaryens can control dragons. It's not exactly a big family secret, but it's certainly not something that is brought up constantly. It's simply a cultural norm for the rulers of Dragonstone.
That said, it's actually quite radical that Jon even exists, because he may one of the first Targaryens born out of a non-incestuous marriage. His mother was Lyanna Stark, which was confirmed when we saw her secret marriage to Daenerys' oldest brother, Rhaegar, through Bran's vision during the Season 7 finale. Now, Jon is drawn romantically to (still unbeknownst to him) his aunt — Daenerys.
As far as we know, Daenerys knows about her family's practice of incest and she most likely knows that she is a product of incest. We haven't heard her comment on her feelings about it during the show, but at the very least it's a topic that's not keeping her awake at night, wracked with worry. While incest is generally considered improper in Westeros, it a social taboo that goes largely untouched by the public; it's simply a fact of life.
The one big wrench in this plan is Jon's strict moral code. He was not raised in a family that had a history of incest nor has he displayed incestuous feelings for anyone in his family (excluding Daenerys here). While Daenerys may have accepted incest as a something in her family's history, even though she had not practiced it before, there's a good chance Jon will be horrified once he learns the truth. This could ultimately affect his relationship with Daenerys, depending on when they find out the information versus how long they'll have been together, romantically.
There's still a lot of to explore on this topic but we're going to have to wait until Season 8 of Game of Thrones to do it. One thing's for certain: the fallout from this revelation will forever impact Jon and Daenerys.