Short Answer: Completely depends on the conditions of the specific relationship in question.
Long Answer: Let's first remember this question seems to be posed to apply to Monogamous relationships. When such a thing such as open relationships do exist, those where people can go out and have outright physical sex with a third party, and it wouldn't count as cheating because it was agreed upon terms of the relationship. So by the same extent, sexual gratification via RP can function the same way. That is assuming sexual gratification is even being had, because in many cases it is quite literally a narrative device, nothing more. But there are cases where people are legitimately turned on by such scenes (I was one of them back when I was 15-17).
And Let's also remember that there isn't just one kind of sex RP. You got your fade to blacks, you get your heavy detailed ones, you got those where it's the main/only focus of the RP etc.
Now, I could go into detail into all the possible RP match ups, perspectives people can have on it etc. But quite honestly, there are so many kinds of RP styles out there and so many different kinds of relationships that I'd end up writing an entire essay and I still wouldn't be done. But in the end it does still boil down to the following:
What is the sexual content in the RP?
What are the motivations and feelings derived from the RP?
Does anything in your past 2 answers state something that you have either specifically agreed not to do, or something you know your partner would not want you doing?
If your answer is Yes? Then you might be cheating.
But that doesn't necessarily mean you're morally in the wrong.
Some relationships are very controlling/constrictive, which once again I'll admit to being guilty of in the past.
What is and isn't seen as cheating is messy business, and people's definitions of cheating honestly vary.
Never will everyone agree on the same term/standards. Weather something is seen as wrong or not has to be agreed upon by the people involved in the relationship.
And if one feels the standards the other is setting don't work with them (May it be too loose, open, restricting, confined, controlling etc.) then it's a good sign you're probably not going to last too long anyways (and if you do last long, you'll probably find a lot of issues along the way), and it might also mean one or both of the partners may simply not be ready for the relationship.
On a note related to the original topic, I find myself wondering how much the responses you've been receiving are biased by the fact that we are in a roleplay community. It might be interesting to see if individuals both male and female who are not in a roleplaying environment—nor who have partners that they are aware partake in such activities—would be equally forgiving.
Note this isn't something I've ran into personally, but I've seen a lot of others claim that friends have confused forum RP'ing with basically "Sexting Online".
So although an interesting idea, I'd personally be too concerned over getting the results muddied by those who have no idea what a forum RP even is.
Um...if you're masturbating to your sex scenes in roleplays than you've definitely got more problems than cheating. To me, writing a sex scene is like reading Fifty Shades of Grey, or the equivalent to it. Words on a page do nothing for me at all, not even the imagery that comes along with it. If anything, I laugh when I write out sex scenes because the euphemisms makes me feel like I'm writing one of my grandmother's Harlequin romance novels. I don't write out a sex scene to get off, I write them out to push my character into developing in some way, whether it's developing feelings for the person their with, or to end up feeling used and degraded when things are over and done with.
If the person on the other side of the screen is getting off on what I write, that's entirely on them, and as long as I don't know about it I don't really care. Is it cheating? Again, not in my eyes, because my goal is for my character, not for me, not for my partner. If you're writing to get yourself off, or get someone else off, that isn't roleplaying, that's cybering. There is a huge difference.
Some people get turned on by the imagery in their head.
I mean just look at any kind of porn or Hentai out there. Originally someone would have had to sit down, think of the idea and go "Oh, that would be hot".
Or if you've ever looked at a person you found attractive, and got turned on and starting having dirty thoughts about things you'd do to them. Same idea in theory, you're turned on by what your mind comes up with, not by what's actually happening in front of you.
Though I will agree, even if the activity in question is seen as cheating by the relationship in question the third party should not been seen as someone to helped cheat if they themselves were unaware of their partners feelings towards the roleplay, and current relationship. In the same way that if someone cheated in real life, the third person shouldn't be blamed, if they were led to believe that they were single.
