Every time Sherlock finds a partner (for romance or just for sex), Mycroft comes along and seduces them so he can talk them into spying on Sherlock for him. Every damn time. And every partner goes along with it, because Mycroft can do all the flashy deducing things Sherlock can, PLUS he's actually got tact.
But then Mycroft gets older, and starts going bald, and gains some weight, and (he thinks) Sherlock starts dating John Watson. And no matter how much Mycroft flirts, John always turns him down. Sherlock thinks it's hilarious and teases Mycroft mercilessly. Mycroft doesn't realize it's because John really is straight - he and Sherlock aren't dating.
(Mycroft says: carpe diem. You just go right on and do that, dude - let me know how that works for ya. John's still going to shut you down.)
Mycroft seduces all of Sherlock's boyfriends
But then Mycroft gets older, and starts going bald, and gains some weight, and (he thinks) Sherlock starts dating John Watson. And no matter how much Mycroft flirts, John always turns him down. Sherlock thinks it's hilarious and teases Mycroft mercilessly. Mycroft doesn't realize it's because John really is straight - he and Sherlock aren't dating.
(Mycroft says: carpe diem. You just go right on and do that, dude - let me know how that works for ya. John's still going to shut you down.)