Sexual and cultural preference does not matter when you're RPing though. STILL not an excuse. You're not having sex with or getting married to your RP partner. They are simply a brain on the other side of the screen. You will probably never see them or even know for sure what their gender is. If you asked me if I was a female I could easily lie. Then you would never know. "Finding it weird" is not an excuse. It's not "gay" to RP a MxF romance if you're a dude and so is your partner. 

The most dynamic, well-rounded and compelling female character I have ever had the pleasure of RPing with was played by a bisexual dude.

What if someone posted a 1x1 interest check with a plot premise I really liked. They seem to be the perfect partner and we agree on every single thing in their rules except one. Let's say I'm a chick and they are requesting a male player to play a male character. But I know I can play awesome male characters. I can match all their other expectations. So I offer to play anyways. I don't mention my gender. They don't ask, they just assume I'm a guy. We discuss the details. We start an RP. We play for months. We never explicitly talk about our real life genders and all my comments about myself could easily refer to a man or a woman. 

Then like a year down the road they find out that I am actually a chick. They accuse me of lying to them and stop RPing with me, even the RP is going awesomely. Not a single other issue has come up. 

Who is in the wrong here? What difference does my gender make? 

Or what if the same thing happens but it's a guy who thinks I'm a girl but it turns out we're both guys? The RP is going great and is exactly what we were both looking for. Why does finding out I'm a guy ruin it for them?

The real reason behind this is that on some level they are conflating their pleasure/satisfaction in what's going on in the RP between our fictional characters with their relationship to ME. At best, that might make them feel weird if I don't match their usual type of romantic partner. But that's their problem and something a good writer should get over. At worst well...there are stalkers and abusers on this site who are simply looking for the right victims and use "RP preference" as a means of selection.