Hi Candy, I am a Bull. I also am very dominant. I am speaking as both a man who has had wives as well as a Dom who has had wives. I assure you, your husband loves you and is very sexually attracted to you. A man who wants to be a cuck may have any number of motivations.
Watching you with another man scratches an itch. The voyeur itch. Watching you with another man, seeing you as a wanton sexual woman. Abandoning yourself to pure pleasure, excites him. A lot. Perhaps like his own personal porn, with you as the star.
He may also feel like he isn't giving you what you need sexually. This is his way to take care of you in essence.
Maybe he has a submissive side. Sharing his wife with an alpha man would feed into that.
Maybe is a combination of things. None are wrong, right, good or bad.
You will read a lot of posts here, most are written by men who have one or more of the above motivations. Rest assured, every one of those men love their wife and feel fulfilled by sharing her.
I encourage you to read about how cucks like to reclaim their wives, perhaps that's something you two may enjoy if you do try this lifestyle out. Remember there is no right or wrong way, just what works for you guys. Talk about it openly and make sure you keep talking about it. That's the most important part.