Obviously, this only applies to someone who you’ve had/are consensually planning on having sex with. Otherwise, it’s none of your business and you shouldn’t ask.
But if you and your trans* partner are going to continue to have a healthy sexual relationship (not that sex is necessary for a relationship, of course), it’s a good idea to ask if they would like to change anything about how you two interact during sexual activities. Things that you used to think were allowed and liked might be out of bounds now that your partner has told you about their gender identity and possibly their dysphoria. Maybe the two of you will try some new things to make sex more enjoyable for your partner.
And if you’re me, maybe your girlfriend will look at you after you timidly ask if she wants to do anything different during sex like you just suggested that she cut off her own leg.
“Why would I want to do that?” We were driving to the mall today, and the heat had us both exhausted.
“Um, I don’t know. But if you did, we could talk about it.”
“I don’t. I like the stuff we do,” she said, grinning. “Don’t worry.”
“I just wanted to make sure you knew that we could-“
“No, I don’t want to change anything! Christ!”
She wasn’t actually mad, she just seemed to think it was odd that I was asking. “Well, it’s better that I ask then keep doing something that you don’t like, right?”
She nodded, and looked over at me with that exasperated smile that seemed to say “You’re so cute when you’re worrying about me.”
And even though I could tell that she thought I was silly for even asking, I was still glad that I brought it up because there are things that she won’t tell me unless I ask. And continuing to have sex in a way that stops her from fully, safely, and comfortably enjoying herself would be far worse than any fleeting awkwardness that my question caused.