I'm in the same situation, and my boyfriend is simply Mr. John. We live together, very committed and plan to be together for a long time, and never get married. My sister's don't even let us stay in the same room when we visit, and they aren't allowed to come over to my house (not a problem, since they are a flight away and I usual fly). My sister's both have girls and don't want them to think a boyfriend can sleep with them in the same room. Imagine a 15 year old saying, we'll Aunt Nancy sleeps with her boyfriend I get it, and who knows I might have even done the same. So I just deal with it, and hope I'm setting a good example for my nieces. If nothing else, I want them to know that when a man/boy really loves you, he doesn't have to sleep with you. Perhaps when they are 20-somethng things will change....
On another note, I think we need a new word for a boyfriend that you live with... boyfriend sounds so young, and not as committed. Partner can make people wonder, especially in the SF bay area where I live. Significant other get so long... Sometimes I just call him my sweetie... but still on the hunt for a new term.... Any suggestions?