I think it really depends on the flirting and the people involved and what each person considers ok. For example, I have pretty flirty conversations with my friends (both guys and girls) with plenty of innuendo, but DF is fine with it cause he knows it's just being silly. NYE a couple of years ago, a friend was drunk and was grabbing the boobs of a lot of the girls and also ended up motorboating me. I thought it was hilarious given who it was so because I didn't care, DF didn't care. Had I taken issue with it though, DF would have aswell. DF could do the same thing, with the flirty conversation etc and depending on who it was, I'd be ok with it. That said, other people wouldn't consider any of that ok. So it really depends on the couple and what they consider ok.
If it was all secret squirrel though, then that wouldn't be ok in my opinion cause it means there's something to hide. I figure if it's all innocent, it doesn't need to be hidden.