I have a high sex drive so would enjoy my partner allowing us to have an open relationship so I could shag some more people. However this is where I think it would fall down.
There is an intimacy that goes hand in hand with a healthy monogamous relationship. It's almost imperceptible as it is so subtle but it is definitely there and the thread is strong.
Directly someone else moves into the 'circle of trust' and becomes intimate with one or both, there is a change. A joke that might have been shared between the primary couple now instead gets shared with the secondary pair. A new 'move' appears in the bedroom, that whilst spices things up, is apparent it comes from the secondary relationship and something deep inside resents that/is jealous.
STDs kind of goes without saying. I assume everyone has had tests? I know the easy comment here would be 'ah, we use condoms' but of course we are all adults, we know that as you get comfortable in a relationship condoms tend to stop being used and other contraception takes it's place. As we are talking Poly relationships I assume the OW's partner is also having other relationships or is liable to? That's a whole other window of STD risk being opened up immediately assuming the secondary primary relationship is not relying on condoms 100%
My main problem with it though is pregnancy. What would happen if the OW (for want if a better word) became pregnant with your husbands child? What would happen if you became pregnant by another man if down the line you decided what was good for the gander ....? I know that would kill me.
There is something very primal about reproduction and it would be an absolute deal breaker if I agreed to an open relationship on principal only to find out a baby is born off the back of it. Mentally I know I would fragment.
So whilst OP you weren't asking for opinions, I have still given you mine as a pretty liberal 30 something year old woman who has been around the block a few times. You are new to this and I'm sure it has the capacity of working, but only if some strong boundaries are set down first. For that I think you REALLY need to find a specialist site frequented by couples who are decades in to this sort of set up.
The reason it's not mainstream is because it is beset with potential pitfalls that could see the end of your marriage. We are not animals living in a herd who get serviced occasionally by the male and then rear our young together with the other pack females. We are complex emotional beings with high intelligence and the capacity to feel jealousy.