Its different for everyone I'm sure, is there ONE reason we love someone. Lots of people are smart and have a good sense of humor, so why do they love that one particular person? I can't answer that really, but as far as being desirable I have a theory, and I wouldn't put this out there if it didn't apply to me in some way, You asked and sorry if this is TMI...
I enjoy Yaoi, which are male/male romantic manga. I have read articles of this, and why its so popular with women. First I want to get some terms out of the way, cause it will will be easier to understand
Seme, "top" but the term originated from martial arts and is derives from the ichidan verb "to attack"
Uke "bottom" derives from the verb "to receive"
Anyway, there have been a few studies now into this, and what they are finding is that
A: There is no gender role that a female feels she has been assigned, she can choose based on the character's personality and by the story plot its self, and most find they identify with the Seme.
B: It allows women to be open about there sexuality. Lets be real here, women are judged by how the dress, right? If she is over dressed she is a prude, and under dress... well.. you know where I'm going. Its the same with what you read, If a woman was out waiting for a bus and reading a romance, I bet she would be approached at least once. Reading a male/male romance that doesn't happen, because there is no box to put her in. Is she straight, kinda... most don't know how to approach it and keep walking.
So after all that I can get to my point, sorry...
What I have found reading into that, is a lot of women are sick the the goody two shoes, vanilla sex role they have been assigned. Look at women's top fantasies, the mysterious stranger, or being ravaged.
I don't want to say he was unfeeling, but he didn't feel things like most people feel. That mysterious stranger isn't gonna get hurt, it can be that raw unemotional sex. The other thing about Sheldon is he was inexperienced, to quote Bernadette " He seemed intelligent, kind of a loner, maybe a little sexually inexperienced, like I'd have to teach him a thing or two. "
I think you're hitting a lot of different things there with one character!