Sometimes it is possible for a secondary to become a second primary. We can plan all we want... You say,
, I feel that I am monoamorous in the sense that thereís only one person (Mr W) that I want to have a house and kids with; to get old with. For me, there are constraints on time and energy for that kind of commitment to other people, and therefore I donít think Iím prepared to spend my life with more than one person (though I admire people who are able to). I am committed to him in a way I could never imagine myself being to anyone else.
But of course, it is always possible another Mr Right, or Ms Right, could come along, who is so perfect for you that you also want to commit to them in some kind of permanent way. Commitment doesn't have to mean living with, or sharing finances, or having kids together... it can just mean a regular weekly date, occasionally taking vacations together, helping them with life issues (like if they need to move house and want help, or fall ill and need help with housework, babysitting each others' kids, etc etc).
It is possible to have 2 people you consider primaries, even though you live with one and not the other.
Then again, if you really want to prevent
committing to another in this way, you might actually need to put up walls yourself in order to keep the relationship on a more casual level. Like you have now... saying you and Sabina can't have sex anymore because you think she trusts you too much!