I think there are too many assumptions in your question.
Doesn't this mean that her husband is forced to spend eternity watching the woman he'd been married to (probably for decades) making out with a man she knew for less than a week, and of whom he'd never heard?
Potentially, yes, if we believe this is some form of heaven and afterlife. But this is no different to a situation that happens quite a lot in the world - people who die having married multiple people. Not, in some of these situations the people have remarried due to a loss of love, hatred, or some other such thing, so it would make sense they wouldn't see their former partners in an afterworld.
But what about people who marry and their spouses die? They then remarry later in their life. Who do they see in this heaven? As we have no idea if a heaven even exists, let alone the dimensions it would function in if it did exist, we simply don't know how this scenario would play out. It's perfectly possibly to truly love multiple people. For all we know, in this scenario Rose could say one last goodbye to Jack, before moving on. Or her relationship with Calvert could have always been about comfort, not love.
There's a plethora of possible reasons for this, but I think the assumption the husband is forced to spent an eternity watching his wife with someone else makes too many assumptions about the structure of heaven and the meaning of the last scene to be valid.
Has James Cameron, or anyone else involved in the film, commented on this apparently cruel idea?
Cameron has always wanted that final scene to be up to the viewer and has specifically not commented in detail on what the scene actually means.
So what do I think the scene means?
For the entire film, we've been told the love story of Jack and Rose. The ending of the film is the culmination of that. There is no mention of Calvert, because he's not been a fixture in our hearts for the duration of the film. We don't know him. We know who he was, as we're told of him, but as a character he's meaningless to us. We don't know his face. We don't know his voice. We don't know his smile. Nothing about putting him on screen at the end would stir any emotion.
However, by including a smiling Jack, and a young, happy Rose, along with the whole crew of the Titanic, we the audience get closure in seeing our lovers reunited.
Whether this scene lasts for moments, a lifetime, or even happens at all, we don't know - and we don't really need to know, hence why Cameron has never really answered any questions on the scene.