I never understood this concept that it's somehow transphobic to not want to have sex with someone who's trans

Here let me explain it to you:

Ask yourself why wouldn't you want to have sex with a trans person? Do you not think that trans women are "real" women? If so >>> you're transphobic. If you actually think that trans women are real women, then why don't you treat them the same? Why wouldn't you want to have sex with one?

Mind you, the example here is that you're clearly attracted to a woman, you want to have sex with her and then she tells you she's trans and you go like "ugh I'm not transphobic or anything but I'm not into fucking degenerates". In this example, you are clearly attracted to the woman and that magically changes because you found out she's trans meaning that the fact she's trans is the problem here. But why is it a problem though? Here are couple routes you can take:

• "she's not feeeeeeeeeemale!!1! I'm a male and I want feeeeeeeeeeemales!!

  True she's not female, but she is a woman. Attraction doesn't work that way. You aren't attracted to a genotype of a person but rather the phenotype ie the look of the organism because your eyes can't fucking scan their DNA to determine if they are male or female. All your brain has to go on is the how they look. If they look like an attractive woman you will be attracted to them regardless if they are male. So sex is largely irrelevant here. Sure you can say "b..but you look for a partner so you can settle down and have children!!1!", no just no. Most of the sex humans do is not for the purpose of reproduction, but for the purpose of pleasure and you can get the same pleasure with a trans woman as you can with a cis woman.

• "she doesn't have a REAL vagina!!1! It won't FEEEL as good

  False. Aside from fringe cases where trans women couldn't afford good surgery, most trans vaginas look exactly like the natural ones. And they feel like them too. Most people wouldn't notice the difference, and you wouldn't either. But lets for the sake of the argument say that she doesn't even have a vagina. She has a penis, a feminine penis but she looks like this, and you're attracted to her. You two find a room and she goes " /u/Surinical I'm a trans woman and I have a feminine penis" What do you do? Go. If you aren't into the whole penis thing, you don't have to do that(daily reminder: genitalia doesn't have a gender. Women can have penises and men can have vaginas. It is not relevant.), you can do other things like : she can give you a handjob or a blowjob or you can fuck her anally. But no! You don't do that because you are transphobic, instead you run away while screaming just how not transphobic you are. Why do I bring up this example? Well, imagine if it's a one night stand right, you won't ever see this woman again, so what is the difference between that and a woman that's on her period and doesn't want to have vaginal sex right now but she's willing to do other stuff with you? Would you run then too? No you wouldn't you transphobic shit.

Gender is performative. Sex is biological. You aren't attracted to sex, you're attracted to gender.

Gender = Man / Woman

Sex = male / female (irrelevant)

The thing with people ITT is that you actually believe you're tolerant and progressive while being horribly transphobic. "lelelelel I don't give a shit if a man in a dress wants me to call him a woman I'll do it!! I'm not transphobic but I totally won't have sex with a man in a dress who isn't a woman even though he identifies as a woman lelellele" - you