Not completely related but I remember having a lot of anger towards certain of my closest friends/family when I came out (as a lesbian) and was faced with the fact that while they were trying to be supportive out of love for me, a lot of them were clearly uncomfortable, and dare I say it a bit ashamed/weirded out by it.
I was only mad for a hot second until I realized that it had taken me 19 years to accept myself so it was maybe fair enough if they needed a minute also. In all cases, coming out (as part of the LGBTQ+ community) is hard and often the anger can be enhanced after coming out when we don’t get the support we hoped for because it’s a reminder that it’s those kind of reactions (or fear of them) that has kept us so uncomfortable and closeted for so long.
To be fair I don’t think your reaction was bad but I do understand how you choosing to hide it from your family, can be hurtful to your sibling. Why not mention it? Does your husband truly needs to understand? personally I don’t ask people to understand my sexuality or to be in favor of it (I 100% do not care if they are or not) I just ask them to accept that that’s the way it is and that therefore their opinion is irrelevant. I am not asking for their opinion or their approval when I come out, I am only letting them know something about me.
So you could just let it be known to your husband without expecting him to « get it » or understand it & just as a simple acknowledgment.
Also I do find your follow up message inappropriate. Saying you are angry it’s coming out of the blue or angry you will never have nephews is definitely something you need to ideally keep to yourself. It’s very rare that people come out out of the blue in their 30’s especially if in a long-term relationship and in a family that’s unlikely to be supportive. It’s a lot more likely that it’s something that they have been through for years and that you simply didn’t notice and that is now finally coming to the surface.
My step dad (who raised me) had a very hard time accepting that I wasn’t suddenly turning gay at 19, because in his mind it was out of the blue when in reality I remember having crushes on girls since as little as 3yo and also equally remember trying to force myself to like boys as that’s what everybody seemed to expect from me. For people who can’t picture someone not being what they see of them it will always come out as a shock to them. For people who live through it, it’s a hard, long grueling process with lots of self-hatred and often trying to force ourselves to change for years.
Something else I found incredibly irritating when I came out was people somehow grieving my straight self (a self that never technically existed). One of my closest friend almost needed support from me to process that we would never have the BBQ on her terrace with both our husbands like she always pictured it, and she really felt like I had burst her bubbles of our future together. As well as people assuming it meant I wouldn’t have kids etc…
That was incredibly strange to me. Like I get it (when you think you know someone but then reality is different etc…) but I found it so inappropriate that they even held me to a standard I might not even have wanted to live up to even if I was straight. Your sibling might never have wanted kids to start with irrelevant of her gender identity. Also being transgender doesn’t mean no kids, it also doesn’t mean you will have to look after her (?), yes you do get to grieve for the sibling you maybe thought you had or wish you had, but don’t put your own (self-created) expectations of them and their future life on their shoulders. Grieve with the people who are going through the same process.
As someone who is gay, I had to spend 19 years of my life grieving the fact that who I was would disappoint some people, including some of my closest and dearest and then I realized I was most disappointed in the fact that those same people caring so much about other people’s sexuality and lives meant that people like me spent years hating themselves instead of loving themselves, which in the end is a lot worse trait than being gay.
So, to answer your OP, you aren’t technically being transphobic, but you also aren’t being as supportive as you think you are. Yes, you are grieving and that’s okay, but also part of being supportive (truly) is to accept that being trans (or gay, or married to someone of a different culture/religion/race) isn’t the end of the world. It doesn’t have to be dramatic, it doesn’t have to be traumatizing. It doesn’t even need to be sad. But what it is will have a lot to do with how you (and everyone who love your sibling) react. If you treat it like you would someone who tells you they don’t eat seafood and just adjust at your level and are supportive without making a big deal of it (one way or another). Then that’s probably the best approach. They are still your sibling, irrelevant of what they look like, are called or dress like. If you would feel more comfortable accepting them being gay then imagine that’s what they have just announced to you and deal with it the same way. Don’t start worrying about their fertility, their relationship, etc… it’s for them to figure out and as a grown adult I am sure they are and have considered it and have weighed the pros and cons of coming out vs not. So just be there for them, as their sibling.