Hello Zone 35.
Welcome.. truthfuly no one should be unhappy about this relationship. However, humans are critical.. children have a different viewpoint because you are the parents and they have a certain image of what a relationship is suppposed to be like. As for friends, conditioning and old negative beliefs come into play and they can be judgmental.. are you wrong for having this relationship? I can't advise you especially since I don't know the real details.
I do encourage people to have relationships for companionship, support and even a sexual encounter if it is what feels right in their heart.. it isn't about how I or anyone else feels.. it is the nature of the relationship, the connection, the soulfulness, the coming together of two who are caregivers and may be filling an important need that is missing from their own individual relationships with their spouses.
You indicate that you care and support hasn't changed for your husband nor his spouse.. how do you feel inside? How does the companion feel? This is what is vitally important.. as for the spouses feelings.. this too, is important and if the relationship is healthy then I can't imagine them being uptight or threatened.. but as the saying goes, different strokes for different folks..
I am asked this so many times, especially from men .. I think as long as you are thinking with your heart rather than other parts, that you can keep it all in a healthy, meaningful perspective..
I hope this assist you in some way.. I feel like I might have rambled on.. but this is it..