I'm not a woman, but I'll weigh in anyway because I'm a rebel.
In my opinion, nope, it's not cheating. Doesn't matter if it's a sex scene in a long RP that isn't just about the sex or straight up cyber sex that's just smut for the sake of smut. Hell, I don't even consider flirting or full on sexting to be cheating. I put all of it in the same general category as checking out attractive people who walk by or looking at porn, neither of which I consider cheating. Calling any of that cheating seems like the same thing as saying someone is stealing if they're looking at items in a store that they have no intent to actually buy. It's nonsensical.
That said, nonsense is pretty much the standard as far as sex and related issues are concerned. People get stupidly possessive and jealous when it comes to whoever they happen to be in a relationship with. This drive to keep competitors away from your mate is one of those natural biological instinct things, one of many such instinctual urges we share in common with damn near every other animal on the planet. While I happen to be able to control or disregard those possessive urges for the most part, others simply are not.
It's all a matter of personal preference and opinion, as most things are. It's best for all involved if people are open about this stuff and plainly ask their partner if they are okay with sexual roleplaying or whatever, rather than hiding it and worrying about it. If the other person isn't okay with it and feels very strongly about it, then you ought to either not do it or go find someone else whose values are a better match for yours. Doing anything else is just stupid and asking for drama and problems later on down the road.
Oh, also, men can get possessive and jealous about their partner doing sexual roleplaying with others. I've actually seen more instances of men getting up in arms about it than women, now that I think about it, so it amuses me that you aimed this particularly at women. I'm guessing that you're a heterosexual male so you were mainly looking for the opinions of your preferred gender, which makes sense, but it made me chuckle nonetheless.