One of them experiences EP and is deeply distressed about it.

It could be Lestrade (as a background to his wife's infidelities, his divorce, his workaholism, etc.) or it could be Sherlock (which would make Mycroft's taunt, "How would you know about [sex]", rather cruel indeed).

The choice is yours, but I'd love to see the partner comfort and reassure his lover that they can work on this together, show respect and affection, etc. Happy ending, please!