Through documentation of their presence and archaeological findings in battle sites, we know that women belonging to the Bushi class fighting in Japan's civil wars was a relatively common sight, particularly during the Sengoku Period. One such warrior was Tomoe Gozen, who fought under Minamoto no Yoshinaka in the Genpei War. Yoshinaka was defeated in the Battle of Awazu in 1184, and there, he supposedly told Tomoe to flee because "He would be ashamed to die with a woman." For me, this incident raises the question on the title. How were Onna-musha generally perceived by their lords, comrades, and the general population of Japan, particularly during the conflicts of the Heian and Muromachi periods, when they were most common on the battlefield?
/u/Morricane and I have talked about female warriors before here. Tomoe Gozen specifically was quite likely fictional. However there definitely were female samurai who fought (and did other stuff, including being leaders of houses and even being a quasi-daimyo, courtesy of /u/LTercero). However in all of these cases, while they were in general praised they were regarded as the exception, meaning the realm of the samurai being that of men was the rule.
Also keep in mind during a siege defense women were very much expected to help, even if in a support role, so many accounts exist of women actually fighting in the defense of a castle. This is not unique to Japan in any way. Tomoe participating in pitched battles is the exception of exceptions, and remember she may very well be fictional.