My husband & I have taken first steps into polyamory. But, I’m not comfortable with him spending the night with his girlfriend. I told him I need time. He feels I’m being unfair. I feel it’s unfair to force what I can’t handle.
I think it’s important that you work out with yourself whether you don’t want him spending the night. Is it because you’re forced to face the reality that they’re having sex, rather than an ambiguous evening date? Is it that you see sleeping together overnight as more intimate than sex? Is it that you really hate being alone at your place overnight?
All those are very different issues that have very different resolutions, so identifying the source and nature of your discomfort is key to working through this bump. Spend some time thinking through this with yourself - make lists, charts, or journal entries; go for a run or meditate; read other people’s experiences - and then go to your partner with the goal of soothing your discomfort rather than controlling his behavior.
In my experience, it can be very difficult to go for “staged” polyamory, or to have rigid rules about what is and isn’t allowed. For them, having a date or sex (which are allowed) might transition naturally into sleeping over (which isn’t allowed), and the distinction might feel frustratingly arbitrary to them. Same goes for any rules like “you can date, but not say I love you” or “certain sex acts are okay, but not others,” etc. Relationships progress organically, and once the ball gets rolling and two people are at the “sleepover” point, it can be hard to artificially stop or re-route things.