There’s only one sure-fire way to avoid being in a consensual gray area in a sexual scenario; use your grown-up words and explicitly ask for consent. If you’re a shy or less verbal person, this might seem like a daunting task.
First let’s deal with some of your concerns…
1. I don’t like “asking.”
People (especially men) have told me that “asking permission” feels weak, that it establishes a power dynamic from the get-go that may not be a total turn on. Can I suggest this adjustment to your logic: the “ask” is a sign of ultimate respect, a new “laying your cape over a mud puddle.” Because who can remember to bring their cape all the time? If your boo is into consent, asking IS foreplay.
2. It makes sex so mechanical.
Maybe there is a small loss of spontaneity. But if you get consent beforehand, or better yet, have a grown up discussion of what you are into and when/where you want to get it, you’ll have the green light to get as weird-as-you-wanna-be when the time comes. I’ll take kink over spontaneity any day.