So I'm assuming if you are married and decide to become poly together then there would have to be certain rules put in place depending on people's comfort level.
Not necessarily. Some people have rules, some do not. Some people prefer to establish their own personal boundaries about what they will or will not accept in relationships, rather than making up rules for their partners to follow or else. This way, they are governing themselves, not policing someone else.
Most polyfolk do have boundaries regarding safer sex and STI prevention, at whatever level they feel they can be comfortable with.
. . . certainly you are more committed to your spouse and mother of your children.
What is "more committed?" One is either committed or not. We all have various commitments in our lives - to our life partners (if we have any), to our employers, our landlords, our creditors, groups we belong to, relatives who depend on us, etc. Do we dole out different "amounts" of commitment, depending on their place in our lives? Nobody says, "Oh, my Tuesday night pool game only gets 20% commitment, while visiting Grandma in the nursing home gets 60% commitment, and my boss gets 90% commitment."
It's not about
how much you are committed, but
what you are committing to.
In western society, we are so used to this strange idea that when we meet someone and start dating, it's only successful if we are moving toward the goal of getting a commitment from them to get "serious" about the relationship and eventually get married, live together, have a family, and all that storybook shit. But that's not the only way to have relationships, not the only way to be committed or dedicated to having a another person in one's life.
When you have multiple relationships and have a commitment to be a life partner and parent with someone, it is absolutely possible to also commit to be a life partner and parent with another person -- although legally you can't marry a second person. But it is also possible that a second partner does not want to build that kind of nuclear arrangement, though they still want a commitment. So, you commit to honesty, respect, treating them well, being there for them when they need it, nurturing the relationship for the long-term, etc., but just not to build a home and family with them. A commitment to one person does not have to be considered more important than the commitment to someone else, even if to the outside world it looks like "less" of a committed relationship for whatever reasons.
Of course, raising children is a huge commitment, and of great importance, but that does not automatically mean that other people don't deserve commitments to them. But it may mean that what you can commit to changes.
I may love the hell out of one girl, but if something about it made my wife feel uncomfortable and it was messing up my relationship with my wife then we may have to part ways for the sake of my commitment to someone else I also love.
What you're talking about here is known as "couple privilege" which
always places one dyad as more important than another. You can do a search for that term here and will find some lengthy and eye-opening discussions on it. If someone is uncomfortable (after giving consent to a poly arrangement, of course), the best thing to do is some inner personal soul-searching to find out why they are uncomfortable and remedy those insecurities internally or through therapy or whatever support system they have - not by limiting what their partner does or does not do with someone else.
Some couples have veto arrangements, whereby they allow their "primary" partner to veto their involvement with someone they are uncomfortable with. This is also hotly debated, you can do a search for that as well. I would never get involved with anyone whose partner has veto power.
Personally, I feel that if someone would be willing to end a relationship with one lover solely because another lover or partner invokes a veto or is just uncomfortable with it for some reason, then neither one of them is ready for polyamory. Why would anyone engage with that person knowing they would be dropped like a hot potato at their metamour's whim? Sure, there are people out there who only want casual sex-only flings and wouldn't care if they were just treated as a "little bit on the side," but polyamory is about developing and nurturing multiple
loving relationships. Not to exclude flings and casual sex partners, because many polyfolk do have casual sex, but if it was all about that, it would be swinging or open relationships, not poly
amory. And if you love someone you don't discard them at the behest of another.
Relationships should only be managed by the people in them. If a person has a partner who is involved with someone else, why should they get a say about how their partner conducts his or her other relationships? That not only negates the autonomy of their partner's other partner, it is also dismissive of the value of that relationship in their partner's life. Utterly disrespectful.