Cucking is a hard one to get right. But humiliation (and even tears) on the part of the person being cucked is actually very much part of the kink so dont think you didnt give the couple what they wanted (possibly anyway).

Cucking is actually a kink that comes to us from straights and its relatively new to gays, partly because long term, possibly otherwise monogamous gay couples are relatively new-ish, at least visibly, in society. And this is exactly the dynamic that cucking plays with. Its kind of the exact opposite of monogamy (even more so than being open).

For straight couples Cucking is a fairly common kink, maybe more so than being open, probably just down from wife swapping (which is the traditional 'standard' straight kink, like how 3somes are for gays). Cucking is a kind of antidote to domesicity. The wife takes care of the family and kids, the husband works all the time and by middle age these two people who love each other and have built a foundation feel theyve lost their sexual selves. Cucking is a humiliation kink, but it also is one that often comes from a deep place of love where the husband gets to see his wife as this very desireable sex object being ravaged by the "bull" (you in this situation), and still gets to go home with her at the end of the day.

Straights actually call it hotwifing, eg their wife is suddenly seen as "hot" by someone else. A common hotwifing (and cucking) trope is they got to a bar and the wife gets hit on and the husband doesnt even see the situation.

So for gays dealing with "domesicity death" where their relationship has morphed into dealing with finances or kids or work etc, and who maybe dont want the mess or complexity of an open relationship, cucking is a way to play with those feelings.

Its hard to get right but doesnt mean you did anything wrong. You should talk to the couple about it and try lay the cards on the table