I'd be fine with being in the same bed with another woman if we were both pleasuring a partner we had in common.
I would NEVER be fine with kissing her or rubbing against her or anything like that. If you want to understand, just imagine it's your brother/sister there. I just wouldn't be comfortable with it, regardless of how much affection I might have for them.
I think most people aren't completely straight (or gay) and would have some room for flexibility in a case like that. But some other people don't and that's perfectly fine. Nobody should be obligated to do anything with anyone else, and "you're already in the same bed!" isn't really an excuse for forcing the issue. For all we know, he already pushed his limit to be willing to be in the same bed at the same time, and they can't be pushed further.