my Father would inroduce us to his parteners and we would have meet/got to know them a few times before any stayed the night.
I think you do have to be carefull when it comes to parteners as to the children it can feel like you are replacing their mother for them as well, particulary if they live with you most the time.
though I was only young when this all when down so might be different for infants or teens
I know my brother had a hard time excepting our now step mother.