I feel terrible about the way I feel

I'm a gay cis male, 20, and no matter how I feel about people, I always want to be supportive of them when it comes to very serious issues. But I'm definitely not perfect, and one issue in particular seems to strike a cord with me; transgenders. I have NOTHING against trans people of either gender, and as someone who's dabbling in being a femboy and talking to a lot of other very feminine guys, it's something I come across a lot- and I've never had an issue with them. But it's the coming out process for others that I know that has a weird effect on me.Just yesterday, I learned a boy I talked to a lot on Tumblr- a femboy "amature model" let's just say- is going to transition to female. He was just posting pictures for fun, and he has a ton of followers, so it's not like I had feelings for him. He was my favorite though, and the whole ordeal left me feeling strange, confused, and even depressed. I feel like I don't have the right to feel that way- I want him, or her rather, to be happy, and I really am proud of them for being brave enough to take a step like that. But I know too, I really am gay; I don't have any sexual desire for females, and I know once they fully transition, I won't be attracted to them. There's plently of cute boys out there, I know, but I think it's all sort of triggered an anxiety in me.Like I said, I'm working on being feminine myself, and my preference in other boys is other femboys, or at least, very feminine types of guys. I've dated a couple- one in particular, though, was VERY feminine, like, you would think he was a girl if you talked to him on the phone. That relationship is in the past now, but I remember deeply repressing a fear while we were together- that he may one day decide to become a woman, and that we wouldn't stay together because of that. He never talked about it, but the worry was there- and I guess hearing about this happening has kind of brought the fear that any other boy I date who acts like that might be considering transitioning too.It's a stupid fear I guess, but it's something that brings two aspects of me into conflict- the one part of me that wants people to be free and to support them all the way, and the other that is inherently homosexual, and wants to be with another man for the rest of my life. I'm afraid any boyfriend of mine becoming a girl would inevitably end our relationship, and I feel like an oppressive douchbag for feeling that way. I feel like an issue like this could be resolved through enough communication, but I don't know how a subject like that could even be broached. Does anyone have any thought?