The only situation where 'practice' is acceptable is when it ends like that:
"By the way, who's the one you want to practice for?"
"Well, duh, it's you of course."
(- And they lived happily ever after...)
I'll go against the grain and say that I kinda dig the idea of "practicing" with a friend who means enough to you to be your friend, but isn't actually the one you love.
It's one of those things I like to fantasize about, but wouldn't ever actually do in real life.
I'm middle eastern so it always irks me when people see sex as such a casual thing that you can just do it with a friend while you're dating someone you love on top of it. Just makes it seem kinda worthless when you do get to do it with your lover.
The whole "practice for the real thing" is a trope I just find hard to accept.
Yeah, regardless of my feelings on cheating and ntr, this is my biggest gripe with those stories.
I can't handle how dumb this is.
The natural response to "it's just practice" is "are you an idiot"?
Nightmare Box is well named for its content, basically one in which both Len and Rin are manipulative and abusive to Miku in different ways, but I certainly cannot argue against the way it handles this question of "practice"