One of the great problems of studying and trying to pattern one's life after the values of ancient Celtic culture is that scant records exist for us to draw from, as well as that much of the records would have been clear to the ancients, but not to us. They knew the context of the message of their stories that we do nopt., I could quote Shakespeare's “star-crossed lovers” and any modern student of English would recognize this as a tragic story of childish love. Not so to ancient readers who wouldn't have the context of the play to refer to. We are stuck with that same condition.
In the lore there are two stories that come to mind, each of the Morrigan and her choice of lovers. One is Dagda and the other is Cuchulain (Cu Cullen). We are told by the lore that upon meeting the Morrigan at the ford, the Dagda made love to her, and did so well she rewarded him with prophecy that he would win the battle against the Fir Bolg. Cullen refused Her multiple times, and he witnessed her at the ford washing the blood off his armor, foretelling his death in the upcoming battle., He would never be king.
In context, these people would have understood the language here that we moderns don't get – the relationship between the king and the land, and the right of rulership.
In the case of the Dagda he sees her at the ford, which is immediately apparent to the ancients as the only place you could safely cross a river. Rivers were highways, sources of water and life, barriers to progress, past and future. The meeting at the ford was a natural crossing-point, and the imagery here would have been clear to a people for whom rivers were all these things. They make love at the ford, and he does such a great job that he is rewarded with the Morrigan's aid against the Fir Bolg. In fact, it is the Dagda who slays their king.
In Cullen's case, he responds differently. He is a Milesian, so the cultural context of a Goddess meeting a hero for a sexual encounter is foreign to him. She announces herself as the daughter of Buan (the Eternal), the king. He misunderstands her advances. In the Tain Bo Cualnge, Cullen is engaged in daily battles with the forces of Queen Medb, and suspects the appearance of a beautiful woman as either a ruse or a distraction. The last thing he needs is a groupie while he's trying to defend against a cattle raid alone, and he tells her he is in no condition to “foregather” with a woman, as he's not eaten and lacks strength. She offers aid, and he says that he didn't undertake the battle to get girls. He's a man operating from his oath, blinded by his commitment and dismisses her.
When she threatens to attack him, as an eel to trip him, his suspicions are confirmed. “More likely that (an eel) than the daughter of a king” he says. This is no princess, but a lying seductress out to stop his plans, probably sent by Medb to seduce him and take him off his game. He'd been chastised once before about a tryst on the day of battle, and wasn't there for the invasion at the border. That wasn't happening again.
The curse then becomes a case of being too proud to admit lack of understanding. And simply not understanding the gift being offered. What is coming from the Morrigan is genuine admiration, as she admired the Dagda. Each are heroes, each do great things, and she seeks to reward heroism and push them to greater deeds.
I've heard claims that Cullen is an example not to spurn a Goddess. I think it's more subtle than that. He commits the sin of mindlessness. He misses the forest for the trees.
So the take away is pay attention, accept the blessing, and understand commitment and honor are good to a point, then it can drive you to ruin. Even a superhero like Cullen can be blinded to what's right in front of him, and cannot proceed to becoming a king when he lacks vision.
The land requires a just ruler, a mindful ruler, a deserving ruler. The Queen will only hold out the crown so long – if you fail to see it, or fail to grasp it, you will be judged as unworthy.