I've just had an interesting conversation with a friend about this.
She's "seeing" someone and while they go on dates and are having sex, they aren't in a relationship yet. She asked him earlier if they were exclusive - he responded to say yes, he thought they already were, and he wasn't having sex with anyone else.
She's talked to him since and it seems he thinks this only applies to sex - IE neither of them have sex with anyone else. He thought flirting was fine. He made an odd comment about did she want him to stop talking to all women on the way out of the door, but did apologise; and he was late for work when she was talking to him (although she wasn't going to get another opportunity anytime soon).
She definitely has feelings for him, and last week he said that he had some feelings too, but he wanted to take it slow and let things progress naturally.
What do you think?