a women having relations (sex included, among other things) is indeed considered a homosexual act, however, this doesn't necessarily mean the persons involved are actually homosexual. As stated previously, they could be bisexual and enjoy both encounters of a hetero and homo nature. The people involved could also actually be bi-curious. Bi-curious being people who are of one sexual preference (homo or hetero) but has a "curiosity" about the opposite (so a homosexual who has a "curious" nature about heterosexual relations, and or vice versa) that they may or may not act upon to commit said acts to play out/satiate this curiosity. As odd as it may sound, performing a homosexual act doesn't mean that you are actually homosexual.
That being said Aran'gar is a bit of a special case since she is LITERALLY a man in a woman's body. Though, as far as has been stated, s/he is actually now bisexual. Though, in the context of the sex s/he had in ToM, yes, that sex would be considered homosexual. There isn't even a grey area to it IMO like there would be with a man who underwent gender swap surgery and then had relations with a woman. Aran'gar is now female, in every aspect, genetically, physically, and even mentally (as evidenced by the fact that she now ALSO enjoys the company of men, which s/he didn't enjoy in her previous male life). So she is female in every respect, it's just that her previous life and memories of it have "twisted" the standard lusts and desires of her current mind.