“There is an entire world between us now, and while my brother is a genius, clearly, and he’s able to understand so much (and so quickly) his social naïveté will likely only be worsened by the consummation of an incestuous relationship. Even this, allowing him to gasp and groan before me as his seed runs over his still-working fist, feels like too much. I wonder if it will be the ruin of me, this little whim of mine. I wonder if it will be the ruin of him.”
When Mycroft says that he and Sherlock have too much history, this is what he’s talking about.