Oh no, many of the bulls include the husband that they are cuckolding in some manner, even if it is just letting them take pictures or watch and masturbate while they are enjoying using the wife's fuck holes.
Besides, just because the wife is enjoying having sex with other men, it doesn't automatically mean she no longer wants her husband. Many women love their husbands even more for giving them the freedom to fuck other men and reciprocate by giving their husbands incredible sex. Many couples find that cuckolding or "hot wife-ing" actually brings them closer together.
Keep in mind that for most, this is just a form of recreational sex, a lot like swinging.
It is primarily about sex, whereas the issues that you brought up about the D/s dynamic and BDSM, while a form of sex for some, isn't necessarily part of the average cuckold or hot wife sexual scene. Most of the people on here are basically having a form of sexual threesome or the wife is just going on "dates" to get laid.
You are asking about a dynamic they don't really get involved with to any great degree unless there is a D/s component in the marriage, such as mine, where the husband is submissive to his wife or where the wife is submissive to her husband and fucks other men at his direction or if the D/s dynamic is between the wife and the "bull". Even then, that only accounts for having a D/s relationship, which doesn't necessarily need to include BDSM.
Many people are surprised to find that BDSM doesn't always have a D/s component either. Many people are masochists without being submissive in
any way. They just enjoy the physical sensations of "bottoming". There are also sadists who have no desire to take charge and be in control of anything beyond what they are doing to inflict pain on a masochist.
Then are are dominants who like to bottom and submissives who enjoy topping. There is no rhyme or reason, it is what it is and there is a someone for everyone, they just need to find each other.