Liberty, I agree with you. I too think the attraction was real on both sides, and I agree that is the reason he took her pulse. She certainly did get to him, as his brother points out, one lonely man desperate to show off, and a woman clever enough to make him dance.
I do think he's a virgin at this point, if only because earlier in the show, Mycroft says in regards to sex:
"How would you know?", and Sherlock, uncharacteristically, has no comeback. I also don't think he's had a lot of opportunities to explore sexually because of what an odd duck he is. "Not really my area," he tells John in ASIP. It's not that women don't find him attractive, but rather that he's not interested in most women; they don't capture his imagination. I really do think brainy=sexy is the key here to his fascination with Irene.
My headcannon also has him hooking up with Irene during his two years away playing dead. I think that by the time we see him again in TEH, he has aged and matured in many respects. He now understands loneliness, as shown by his conversation with Mycroft in that episode, and his ease in developing a relationship with Janine in HLV is otherwise unexplained.