Indeed, the terminology is a bit misleading. I guess you meant MFM relationships in general, not the special kind of polyandry (married to many men).
I can't say that this has been the sole problem when someone was coping badly with the news that I have a husband and a boyfriend but especially for the older ones it was some kind of 'claiming the male role' in this relationship constellation. Those who were OK with our relationship haven't had any problems with the fact that it was a woman having more than one man by her side. Therefore I guess that if you have a problem with the relationship you will likely cling to 'old fashioned' ideas more easily, because that's usually where your general problem lies in this case
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.