I mean, you're right, but I think it's a little more complicated than that.
I don't think it's just about hating on us; what they're saying is not "men don't deserve periods" or whatever, it's "periods make me a woman. periods are About Women. that is important- crucial, even- to Woman Culture, and must be preserved."
When trans men talk about periods, the response from radfems is either outright transphobia, courtesy of TERFs: "of course you feel this way, you are A Woman and this is just proof that you're deluding yourself by thinking otherwise"
Or a more quiet form of transphobia, courtesy of radfems who believe themselves to be trans-inclusive: "When you are involved in this conversation, you must recognize that you are in OUR territory, discussing OUR culture. If you reject womanhood for yourself, either you give up on talking about this or you accept that you are a woman when you do."
We might be "allowed" to identify as men, but they still view this as a kind of culture or signifier issue. They view it as dangerous when we ask to discuss periods while maintaining that we are a man, and that this is a Man's Experience. We're diluting and confusing Woman Signifiers, and weakening Woman Culture.
They believe that an essential part of making women strong, or protecting women, is this kind of culture purity and shared group culture. This post does a great job of explaining it, but the logic is, essentially, similar to that of a race nationalist: our group is in danger, and the best way to protect it is to unify under a shared culture and segregate from those who wish to hurt us. They believe their culture is a shared, immutable thing; outsiders can only create insulting facsimiles- lacking a true understanding- and sharing this culture would weaken the strength they find in the purity of a shared identity.
Trans men are often called "gender traitors", or considered to be as much; it's a staple of transphobia against us. You can see this logic come through here: we rejected our culture, defected to manhood, and in doing so we have threatened the purity of Woman Culture. In order for them to maintain this ideology, they must choose to believe one of a few options:
1. Denial: trans men are just women, we will never not be women, and we're just deluding ourselves.
2. Rejection: trans men have made our choice, and we've chosen our side; now we need to accept that we are no longer a part of Womankind, and give up all access to those spaces and that culture.
3. Humoring: trans men can say we're men, and that's fine, but when we discuss Woman Culture we must recognize it as Woman Culture. For the purposes of that conversation, we are still women, and the space belongs to them.
When we insist that:
1. We are men,
2. We are men with "women's" experiences, and
3. Those experiences do not belong to women, because they did not happen to a woman, because they happened to us and we are men,
We're telling them that they must give this thing up as a part of Woman Culture, they must understand that this does not belong to them, and we're asking them to invite men into Women's Spaces.
They often don't think of themselves as hating us in order to believe this; to them, there are ways here in which we can still participate in this space. The problem is our insistence on all three of these points, and the hatred and violence comes in when they realize we won't compromise on any of them.
But the thing is, like, this isn't actually respecting trans men's identities. This isn't "trans inclusion", it's "tolerating trans people as long as they don't disrupt our understanding of the world". And, of course, you can't "love" trans people while simultaneously telling us we're wrong about ourselves and must comply with what you think is best for us.
This is the part of radical feminism that's hard to effectively argue, because it goes against the core of that ideology. But here it is, for the radfems who might see this:
You do not need a Pure Culture in order to be safe or strong. Trans people are your allies, and breaking down rigid gender binaries challenges and weakens the patriarchy. This is good for feminism! We want this!
And, you know, you can still understand things like this to be a part of your womanhood without them being solely for women. You can still understand them as things patriarchy has targeted for an association with women, and also understand that transphobia means trans men's periods are not seen as a Man Thing by patriarchy, even though we are men.
And that is a core part of our relationship to periods, too; we often feel dysphoric about them because of the patriarchal association with womanhood, and we are often put in danger of being outed as trans because of them; or denied resources cis women would have access to, because "men don't have periods".
You can still understand periods as something that is part of your relationship to womanhood, without that being true for everyone who experiences them. You can still understand your womanhood to include periods, without that being true for everyone who is a woman. Your relationship to gender can be unique and individual, and so can everyone else's.