Some pretty clearly implied sexings going on in this one (nothing explicit but still...), as well as me making Sherlock miserable for once, instead of John. Read at your own discretion.
Sherlock eyes the packet askance. "Condoms, John? You know I'm… I've… This is new to me."
John smiles fondly at Sherlock's vulnerability. "Yes, I know. But I'm not. Combined with your history of drug use, it's just safer." He pauses. "If you enjoy this, and want to keep doing it, we'll get tested, but tonight I insist."
John takes his time prepping Sherlock, and when he finally sinks in it's glorious. He's lost in the moment until he realises that Sherlock is squirming in discomfort, not pleasure. John stops abruptly.
"Is something wrong?"
"It's... nothing. Itches. Keep going."
"Sherlock, we're not doing this if you're uncomfortable. It's fine. Let me see."
With great care, John pulls out and reaches down to examine Sherlock. He's treated to the sight of livid red hives in places hives have no right to be.
"Sherlock! Why didn't you tell me you were allergic to latex?"
"Oh. Is that what that is?"
John cringes. "How can you be in your thirties and not know this? What about the gloves in the lab?"
"Bart's uses nitrile. Just never came up, I suppose. Jooohnnnn. It stings. Do something!"
"Okay, Sherlock. I'm going to run to the chemist's and pick up some antihistamines, and some ointment. In the meantime, go sit in a cool bath."
"Hurry, John. It's burning!"
So, they say to write what you know. I'm allergic to latex. I found out when I was a teenager. You do the math...