Have you looked into swinging? There are whole communities for this.
Also, I would say, it is ok to have the aim of opening for casual sex, but I would advise couples not to make RULES against feelings. I would advise people to try to be easygoing about it...and realize that what we see as "a relationship" is usually a whole LOT of pieces bundled together. Doing ethical non-mono, often means unbundling them, and deciding what you can do, and what you can't.
So when it comes to things we call "escalator" (the "relationship escalator" is a term meaning all the milestones most people try to hit as they progress through a relationship, to include dates, first kiss, sex, meeting family, moving in, etc etc)....the escalator stuff, you can say you're not up for that with other partners. Right, you're not looking for any brother-husbands, sister-wives, or unicorn-thirds, to move in and be part of the family. That is fine, and it makes sense to disclose that to partners, so they know that if they are seeking that, you're not a good match.
But saying "no feelings allowed, only sex" is often a bad idea because feelings can creep up on us. They often happen whether we want them to, or not. And getting mad at a partner for "breaking" that rule, is...unproductive.
However, feelings, even stated love, does NOT mean that anybody has to ride the escalator. That is so important. Love isn't a trap!
Personally, in a situation like this, I'd go for sexual variety, but be open to warm feelings, even intense feelings if they happen, but take a view that whatever "relationships" (again, NOT escalator relationships--nobody is picking out china here!) form, will run a natural course for however long and evolve as need be. In my experience, either the outside partner will eventually want something more serious with someone more available and they'll move on, or else the thing will evolve into friendship, or it will just sort of fade out. Doing it that way is easier, and probably safer, in my opinion, than just having sex with people you barely know and meet just for sex.
Feelings, and love, are just by-products of getting to know and appreciate another person. It's nice to know and appreciate the people that we are intimate with. I'm not that into sex without intimacy, I find it lacking and boring. All this is just my long, roundabout way of saying...don't be scared of feelings, or "relationships." You get to write the script however you want.