Basically I found it necessary to sit down and figure out some rules because my YF is becoming more affectionate and more curious at the same time, and if something comes up I need to be able to respond quickly rather than hesitating.

There's actually more to this than temptation, another concern is the outward appearance. I'm convinced I could share a bed with him without anything happening, but I don't think it would go over well if I word got out. Exceptions might come up, so I'm not really setting a firm rule on it, but in normal situations where I'm staying at his place or he's at mine, I've decided my response to "you can sleep in my bed" needs to be "no thanks", instead of "uhh..."