I think it kind of smacks as someone who wants to appropriate a struggle that is not their own.
I think most people have known a girl who claimed she was bisexual because she found some actress attractive and she wanted the attention from her friend group but found the idea of sex with women as disgusting and so never ever had a relationship with any women.
It wasn't too long ago that being gay or lesbian meant you had to hide who you were or who you loved and a lot of older people had to live through that. Some people still do to avoid losing their family. A lot of younger people don't understand that. So seeing someone who enjoys the privilege of being straight and being able to openly love who they want pretending to be marginalized just feels kind of gross. Kind of feels like Rachel Dolezal. You can look her up.
I'm probably going to be downvoted for this opinion but we were asked to explain why those around OP thought their deception was bad. This is likely why.
I understand that the climate today is vastly different than what it was when I was growing up. These days being gay or lesbian doesn't necessitate a struggle and that's fine. I am coming to terms with the fact that it's more important to be okay for people to self-identify these days, despite what I or others may think, since being queer is simply far more widely accepted (in the US). This is true for being the LGB portion of being LGBT to a large degree, but we haven't gotten there for the trans portion.
I'm just explaining the perspective people who have issues with OP might have and thus why they might have a problem which is what they asked about. Especially as someone who grew up in some of the hard days (i.e. being disowned for being gay, having to fight for my rights to marry my husband, etc.). And also experiencing people in my own journey that claimed they could speak on my behalf or for LGBTQ people as a whole despite having almost no understanding of the struggles we had to go through in the past. The struggles the gay community has faced in the past are being experienced right now though by trans folk perhaps even to a larger degree. Which is why it is more likely to be found problematic to pretend to be trans when you are not.