You have me slightly on the confused here, I've 2 points to address, in 5 parts (HAIL ERIS)...
1. Sex talk
2. Flirting (online or off)
3. Taking another person's phone number (opposite sex) and HIDING it.
4. Thinking of another person while having sex...yeah not so good.
5. Obviously, any sexual act with anyone else
6. kissing someone else (and I don't mean the peck on your family member)
7. online correspondence (especially e-mail) with someone you are hiding from your partner
8. text messaging that same type of person through your cell phone
1...'sex talk' must be defined in order to even begin this discussion...there are many kinds of sex talk...talking about sex in a biological, instructional, or anecdotal manner, not only ok & cool, but, sometime NECESSARY...talking like a phone sex operator, not right to say the least (that may be what you mean)...then, there is the 'grey area' of the off colour jokes & silliness to which we may thank the likes of Benny Hill for...I find it very hard to pass up a good-pun-fer-fun (but, you may mean this as well.)
2...again, there is the Benny Hill type of humorous comment that was not meant to go anywhere...then, there is the flirting that is expressly the prelude to mating rituals, which, may or may not be acceptable, depending on the cultural taboos that the partners agree upon at the onset of the partnership.
3, 7, &8... really the exact same thing to me, 'clandestine communication'...ANY thing that is hidden, destroys the root of the word 'partner'.
5...duh...
6...you mean on the open mouth, right?
...Oh, as I started this way back before he posted, I type slow when I have to think up such bombastic words, whilst cleaning the house..really slow... +1 to MSTERBEAU for saying that SO CONCISELY!