Sure wasn't political on Robert's Side being he called her name while humping Cersei and the first thing he did when he got to Winterfell was visit her tomb. And he is known to be voracious in his sexual appetites so... ya know.
And Apparently the Stark Dark Hair Gene is not that strong:
If Lyanna had coupled with Robert and there was even the remotest possibility that baby Jon was Robert’s son, then it would make more sense for her to just say so. Ned, this is Robert’s son - look at his hair and please protect him! Robert wouldn’t have a reason to kill his own bastard.