Well it's probably better to say the problem with it rather than just say it's transphobic.
The reason some people are having issue with this post is as follows:
The problem with your post is that you're claiming that transgender people should simply accept that they are the sex they were born with and should just move on, but this has proven time and time again, throughout the vales of history, through monumental research by numerous psychological and medical associations, to not be so easy. What I find interesting is your post sort of contradicts itself, you talk about people wanting to be fitter and you say "Go do it!" As if the only way to make yourself feel good about being fitter is to actually become fitter. Yet, when it comes to transgender people who want to be a different sex, you instead say "Just accept it!", why would you use "fitter" as an example to prove your point when it actually contradicts your transgender point? I dunno.
Either way I feel like there's a counter culture brewing. Basically a lot of people believe that transgender people are going over the top but what's interesting is lesbian and gay people did this exact same thing back before 2000 and it basically has followed the same pattern. People were like "I don't mind if people are gay but if they're gay in public that's sort of gross" and "Bleh, these people waving gay pride flags and stuff, like can't they just accept that they're a male who should be attracted to females?". Anytime a culture becomes exclusive, anytime a culture becomes prominent to say "We have this property that you do not have", it seems a counter culture develops.
In a way I can see why, some transgender people are outright hostile to cis people, and it's hard to be friendly to a community that you perceive as against you. This whole idea of "Checking your privillage" was supposed to be about recognizing the hardships that transgender peoples face, but some people these days seem to use it more as a "You should feel guilty for not being born trans", which to me is totally missing the whole point. Another thing I don't like is when transgender people say you can't have an opinion on transgender people that is against their opinion, like, they build off this entire idea that their opinion is protected by the social idea that you shouldn't be mean to transgender people or otherwise they'd get more upset than a usual person, and I think that's not a very good stance to have.
Regardless, these things I'm specifying are not the majority of transgender people, the majority of transgender people are very nice, friendly, and accepting. They respect you if you respect them and they just are generally really cool people. The problem is that the minority of transgender people are very loud, and very noisy. It's sort of a much, much less extreme case of Daesh and Islam, in that Islam is clouded out by the loud minority of Daesh people, of course these transgender are no where near as worse as Daesh, it's just highlighted the fact of a minority clouding out the majority.
So here's my final point. You may not understand transgender people, I recommend doing some research. You may find some transgender people are doing stuff you disagree with, maybe just don't hang out with those particular transgender people then. Just don't generalize, cause a lot of transgender people are cool people and they're affliction is pretty real. Don't be mean to transgender people, not because they're transgender, but because they're people.