I think certain ways of dating and certain behaviors are more likely to bring on notions of love. To me, people that will frequently have very nice dinner dates 1 on 1, especially if they are newbies, and then 1 on 1 all night with him, breakfast together, etc..... that's increasing the likihood of deeper bonding. I don't just think it's insecure people that fall in love with bulls, though those types are probably at higher risk.
Also, I don't think there needs to be any gate keeping and preaching that a couple's cuckolding relationship can't evolve into a form of polyamory. I think polyamory is more complex and slightly more risky for the couple, but couples can go that route.
The bigger risk is of love starting and her being set on being the type that loves just one person. Then, she may feel that she has to choose. The same would go for risk coming from the bull. Even if the couple can navigate her expressing love for the bull, he might start getting ideas of slowly pulling her out of her marriage because her emotions gave him a sign she could possibly make that change.
To me, a poly-ish situation works best in cuckolding when people don't try to live together, and love isn't expected to be equal in all directions, and the marriage is held as most important by all 3 people. I think there's some long standing cuckold threesomes that are kinda poly, but they never use that label because they are more traditional type people.
To me, if people want to decrease chances of deeper romantic love, they shouldn't go on nice romantic outings 1 on 1, and don't sleep over unless you're a pro at keeping it to the friends with benefits level, and then still not often.
I think the frequency of dates factors in too.
Full disclosure, I was a lover to a married poly woman for 6.5 years (I'm more of a cuck than a bull though). To me, there's pros and cons to the swinger route, and to the poly route.