My metamour is moving in, and there's tension because my partner really want us to date

My partner's girlfriend is moving in with us and I'm a bit nervous because while she and I are good friends, we are not romantically interested in each other. That's okay with us but my partner really wants for us to be romantically involved and can't seem to let it go. Is this going to end in disaster?

The co-habitation isn't the problem, it's your partner being pushy about this. They need to drop the issue, and yesterday. Sit them down and explain that you're very nervous about her moving in, not because there's any issue with the dynamic between you two, but because of your partner not being willing to let this go. Tell them that they can't wheedle, cajole, argue, convince, or otherwise mind-trick two people into dating if they don't want to date each other, and trying to do so will only cause problems. If they can't accept this or refuse to stop, you need to decide whether this is a situation - the relationship and/or the living arrangement - that you feel comfortable staying in.