Mycology

It's commonly regarded as sexist in most contexts, at least that is / was my understanding. The thesis reiterates the harm outside of a workplace setting:

This suggests that the infantilizing label girl has the most impact and is most harmful in contexts where qualities of maturity, leadership, and adultness are most critical, such as in workplace and leadership settings. I do not mean to suggest, however, that being called girl in a non-workplace setting is unproblematic, as it is possible that such labels could have a cumulative effect over time in any setting: the connotations of naivety and innocence may take their toll on women in the long-term.

While referring to a man as a boy likewise has problematic features (esp. as a racist slur, like when a white man referring to a black man as "boy"), I think it is considered more harmful to refer to a woman as a "girl" due to the context of women being marginalized historically and presently by undermining their sense of personal autonomy and authority (e.g. the way Hegel described women as akin plants, or the way Aristotle argued women are natural mutilations and aberrations of the proper male form who do not exercise their will and require men to manage their affairs for their own good, the way parents must for their children).

This is all contextual though - women peers referring to one another as "girl" can be affirming or positive without the infantilizing meaning or impact, so part of the problem is the context of a man referring to a woman as "girl".

And of course you probably didn't intend this, or even been aware it has any sexist connotation, in fact I suspect this kind of behavior is rather common (lots of men can be clueless about the subtle differences in language and the impact it can have on women).

This is somewhat touched on in the article as well:

Indeed, this study may be the first to show that a commonly used label for a group of people (and one that is even preferred by members of that group in many contexts) can have a detrimental effect on members of that group. Previous research (e.g., Boeckmann & Liew, 2002; Carnaghi & Maass, 2007; Evans & Chapman, 2014; Leets, 2002; Leets & Giles, 1997) has documented the effects of hate speech and overtly derogatory labels on minority group members. The term girl reveals another insidious type of language effect that passes by relatively unnoticed and is deemed “normal,” yet has deleterious consequences.

By the way, I want to be clear that the take-away here isn't that you're sexist or bad, the take-away should just be that many women feel infantilized by being referred to as a girl and that it's good to be aware of and sensitive to that.

Besides the harm it can cause, it's also probably just pragmatically useful for you to know that other women seeing you refer to another woman as "girl" might have a chilling effect or even spark anger, since it is seen as sexist. I think the context matters here, but a lot of women are victims of physical and sexual assault on top of all the other ways they can be marginalized in this society, and the resulting trauma can cause outsized / disproportionate responses or outbursts. It's just worth noting that if someone seems to be overreacting to something you don't see as that big of a deal, there might be deeper issues there. I don't want you to feel like you have to walk on egg-shells, but it's also good to be aware and empathetic if you are willing to.