He described it as "a consensual workplace relationship". That could be an implicit reference to an open relationship, and him engaging in a situationship with Alex with his wife's consent.
You can choose to interpret that in any way you want, but you also need to acknowledge that he and Ariel continued their marriage together and, as previously mentioned, he saw no need to keep the relationship with Alex a secret as they would go for meals and drinks together.
I'm not looking for an argument, but I am just saying I don't think the whole situation as clear cut as the media circuit wanted us to believe. Fans had got way too emotionally invested in his relationship and acted like they were the scorned party when Ned's actions came to light.