Back in the day when the internet was new and most of my system RP partners were goth girls playing femme vampire guys.. yeah, I could totally tell when it was someone not familiar with being a man was writing their "man fanfic". I loved my partners and enjoyed playing, but the contrast with the occasional male partner was obvious.
After a very long break and finding these sorts of non-system forums, it's been rare that I could really the difference between a male or female partner playing a male character. If someone states their sex/gender it in their RT, I forget it immediately. I never ask. I never bring it up in OOC. I'm absolutely ambivalent.
I appreciate that for female players, there can be a sense of threat or power dynamic with a male partner. I've experienced creepy male players that I ended up blocking. I respect if a female player prefers female partners. I don't think I've been declined for being male, but that's probably because a gay dude is lower risk of getting creepy with a female partner. Regardless, I respect that players have comfort preferences.
Interestingly, I've had a large percentage of partners eventually tell me they were trans. I had no inkling while playing, but in hindsight there were sometimes a few hints IC, but nothing that was exclusive to trans partners. *shrug*