I'll try and answer this question honestly, as an actual feminist, as most people in this thread seem to employing the straw feminist argument. Hear me out.

A lot of men who talk about heightism frame it as an issue of being unattractive to women, not being able to get dates, not being able to get laid, etc. This is not an issue that is going to gain much traction with feminist women, especially when some men talk about it in such a way that shows that they believe they are somehow owed sex - and that is DEFINITELY anathema to feminists.

Feminism is about the advancement of women. The issues most contemporary US feminists care about are rape/sexual assault, reproductive rights, pay equity, domestic violence, and media representation. Feminists also tend to be intersectional and care about things like LGBT rights and racial justice. What I'm trying to say here is that if a man thinks he isn't getting laid because he's short, well, that doesn't fall under the feminist movement's purview. It's not at the top of our agenda when women are getting raped and colleges won't do anything about it, when the government seems poised to take away our right to an abortion, when black trans women are getting murdered at alarming rates.

I absolutely agree with you that height discrimination affects short women as well. I have experienced it myself - by virtue of living in a small, female body, I am automatically considered young, weak, unfit for certain activities. I am condescended to and belittled for my height. I actually think a feminist analysis of heightism (as it affects all genders) would be useful and cool. But that wouldn't gain a whole lot of traction in a community like r/short because of the attitude towards women here, and on Reddit in general.

I think it sucks that short guys get discriminated against, even in the dating world. But no, advocating for short men is not part of my feminist practice. Sorry. (I make up for it by fucking short guys) My advice is to keep advocating for yourselves, keep supporting yourselves, but don't be surprised that the feminist movement doesn't take you seriously when you seem to be virulently opposed to feminism - and maybe women in general - from the get go.