I think that tumblr post is very, very intriguing. A part of my mind wants to believe the notion Mycroft has indeed had sex to "fit in" with his peers. Fit in, isn't exactly the word I'm looking for, but close enough. How he is viewed before his peers appears to be very, very important. The way he says the line, it comes across a little smug...making me believe that yeah, he has. He's yet again, one-upping his brother in their battle of wit and I guess, experience. On the other hand, however, I believe that in order for you to want to have sex you must have an attraction to your partner, or you know, it doesn't work. Mycroft nor Sherlock appear to have an attraction to anyone, at least when it comes to outer appearances. Sherlock does say that he can see beauty but I don't think it's the same way that we may think. I think a person can find the beauty in a painting, a flower, or a sunset, but we won't be attracted to it, if that makes sense. I find it hard to imagine..either one engaging in a sexual act, even if it were to fit in with their peers.