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The first time Jim gets his mouth on Spock's cock, his lok, he's surprised by the flavor of Spock's pre-cum.
Nothing in any of the anatomy or physiology books Jim's read – or the erotica novels he's stashed in a poorly labeled folder on his personal pad – have suggested vulcan semen or pre-ejaculate was spicy to humans, or even just perceived as such.
Still, Jim developed a taste for a bit of heat as a pre-teen, and he can't say he minds the flavor now. It does make it a bit harder to swallow when Spock cums in his mouth, but he will be the first to admit that he loves a challenge.
Slight burn lingers in Jim's throat as Spock tongues his way down Jim's chest to return the favor, but that could just as well have been the deep-throating he'd been attempting.
By the time Jim wakes the following morning, his throat is no worse for wear.
----
Jim isn't surprising when the spill of Spock's cum into his ass burns as well. As any a human who's consumed a reasonably spicy meal can attest, the delicate tissue of the inner wall of the rectum can perceive the burning sensation of capsaicin just as well as the tissues of the mouth.
Spock fingers Jim's fucked out hole idly, as Jim attempts to catch his breath from the spectacular mutual orgasms.
"You're unusually flushed here." Spock notes.
"A good fucking will do that to you." Jim hums. "I'm happy to return the favor next time."
Spock doesn't let up his idle motion at Jim's hole as he says, "I may be amenable to that activity in the future." Which is basically Spock-speak for, 'sounds good, next time you can top.'
Jim's totally winning at this 'fucking his first officer' stuff.
---
"You are certain this is normal." Spock asks – well, he states it, but Jim can pick up on the hint of a question in the raise of his eyebrow – some half-dozen or so days later, his fingers once again playing with Jim's burning hole, as they both bask in the afterglow.
"Looking all hot and bothered after my sexy first officer rails me?" Jim smirks. "I'm pretty sure."
"The flushing is more extensive than I have noted in previous partners on the receiving end of anal intercourse."
Jim groans. "Mentioning past partners like that is not sexy, Spock."
Spock ignores him.
Jim sighs. "It's probably the capsaicin or whatever you have going on in your cum that makes it spicy. Increased blood flow is a pretty normal human reaction to spicy food."
Spock goes very still.
"What?" Jim frowns. "What's wrong, Spock?"
"Jim." Spock says, slowly. "There is no element of my anatomy or physiology that should be perceived as 'spicy' by the human body."
"You sure?" Jim frowns. "Maybe your past partners just never mentioned it."
"I am quite sure several of them would have mentioned it."
"Huh." Jim says. Weird. Not the weirdest thing Jim's body's ever done, but still weird.
"Jim. I am correct that humans sometimes experience foods they are allergic to as having a 'spicy' flavor, when the food item itself has no such properties?" Spock's face is doing the scary eyebrows thing again.
"What? You don't think I'm allergic to your cum, do you? Man, that would be so lame." Jim drops back onto his elbows, that he may more effectively bury his face in Spock's pillow.
"I would prefer," Spock says, " if you would refrain from preforming fellatio on me until we can disprove the hypothesis."
"Lame." Jim groaned into the pillow. He lifted his head a fraction. "What if we use a condom when I give you head?"
Spock hummed. He finally removed his fingers from Jim's (still burning) hole. "That would be acceptable."
"Yay." Jim deadpanned.
Spock finally laid down beside him to cuddle.
Jim fucking hates having allergies.