These are not common expressions (at least in American English) Most likely an American speaker would say those words only to avoid saying a word like ‘lover’ or ‘sex partner’ even though the same meaning is implied. More common terms I hear in the US are: Girlfriend/boyfriend (which doesn’t imply sharing a bed, but might) Lover (implies someone you are intimate and romantic with) Partner (implies you are in a committed relationship even though perhaps not formally married. ‘Partner’ is also a way of avoiding saying whether your partner is male or female) Housemate (means you live in the same house, but are not romantically involved) Roommate (Literally means you share a room- but this has a double meaning: you might be students in a dorm, not romantically involved, or workers sharing an office. But it could also mean someone you live with and share a bedroom with. So calling someone your roommate doesn’t make the situation clear. I hear people say this on purpose to avoid calling attention to the romantic nature of their relationship. ) Bunkmate (means you share bunk beds in a dormitory, like in the military. Otherwise same as roommate) Significant Other (use this to talk about someone else’s partner without saying exactly what the realationship is, for instance at work where you don’t want to talk about whether someone is married or not: ‘Bring your Significant Other to the party!’ This phrase is awkward and bookish, but it is used that way a little bit on purpose.) There are also some other colloquial expressions you might consider: ‘Friends with benefits’ means someone who is just your friend but whom you have sex with for fun. ‘F*** Buddy’ means the same. You can guess what the four-letter word is. This term is coarse, but honest.

Both of these words are colloquial. Bed mate is someone with whom you just share your bed and Bed partner is someone with whom you share everything(hope you understood)