Some people will consider you sexist.
There hasn't been a huge amount of discussion on domination/submission in the context of online roleplaying specifically but I'm not sure there's much difference between online play and "real life" play in this context; in an online roleplay you're consensually interacting with someone else, in offline "real world" play you're consensually interacting with someone else, it's just that instead of typing what "you" (or your character) would do, you're actually doing it. And BDSM in the real world has frequently been accused of sexism by certain schools of feminism. I'll note that I'm discussing BDSM in general here rather than strictly dom/sub (one can have a dom/sub interaction that is basically "vanilla") although the two overlap.
As a general rule these criticisms tend to come from the more radical side of the spectrum and there are several arguments they present. The "All PIV is rape" brigade (i.e. all penetrative sex, consensual or not, is rape) fairly obviously consider it rape; their main argument for why PIV is rape is that penetrative sex is inherently violent and sexual violence is rape/abuse so BDSM, which includes a lot of sexual "violence" is clearly rape/abuse and thus by fantasizing about BDSM one is fantasizing about rape. Then there's the school of thought that says that women aren't really
consenting to BDSM play; they're victims of "Societal Stockholm Syndrome" (a theory advanced by Professor Dee Graham of the University of Cincinnati in "Loving to Survive") and as such their consent is invalid, meaning that your supposedly consensual roleplay is actually abusive and thus rape and thus sexist. There's the school of thought that argues your fantasy to dominate women is one that you've been conditioned to have by a society that is inherently sexist against women and suggests it is a man's right to dominate women and thus it is sexist. There's the school of thought focusing on BDSM and its place in rape culture where engaging in BDSM, even consensually, presents an image that it's OK to abuse and dominate women which gives rapists and abusers encouragement and the vision that it's OK. There's the school of thought that views BDSM as a micro-aggression; regardless of your actual views the very fact that you fantasize about dominating women is going to make women uncomfortable around you and suspect that you won't take them seriously/view them only as sexual objects and you're thus being sexist. There's the school of thought that says that your fantasy is dominating women, not dominating a consenting
woman, and is thus inherently abusive. Then there's the school of thought that looks at the fact your fantasies are primarily about dominating women but not men; as you don't want BDSM play where you're dominating a man it's the gender that's the key to your fantasy, not the domination and thus you're being sexist... using similar logic, although reaching a different end-point, to the arguments that hold if you're not attracted to trans-folk then you're transphobic, if you're not attracted to overweight partners then you're fatphobic and if you're less attracted to a certain race (or more attracted to one in particular) then you're racist. There are other objections to BDSM from a feminist perspective out there, but that's just a quick overview.
Maybe there's some truth to some of those arguments. Maybe my kink for domination has been ingrained on me by society rather than reflecting my true views. Maybe some people in the "real world" would think I'll view them negatively if they were aware of my fantasies. Maybe my enjoyment of BDSM play as a dominant reveals some deep inner truth about me where the "consensual" part is just a fig leaf I use to give me some moral justification. Maybe despite the fact that I've engaged in BDSM play with all genders the fact I primarily do so with women gives an indication that I want to see women suffer and that I think it's right for them to be submissive to me.
But so what?
Almost regardless of your actions they'll be someone out there who thinks what you're doing is sexist... see the "All PIV is rape" people mentioned above. But you or your actions being called sexist isn't the end of the world, nor does it mean that your actions either 1) actually sexist or 2) bad. In a world where a man spreading his legs is an act of sexist domination, air conditioning is a way for sexist men to keep women in their place and well, that these are the most misogynistic moments of 2015
(rather than say the Islamic State's penchant for sex slaves, women being stoned to death for adultery in Afghanistan or the fact that it's still nearly impossible for a woman to open a bank account or drive in Saudi Arabia), the term "sexist" has been largely devalued anyway.
What matters is how you treat and interact with people. And if sexual fantasies do represent some wider truth as to how you view women then it still doesn't matter as long as that part of your world view doesn't come to dominate the rest.