Someone else here mentioned that Bill Gates met Melinda (his wife) while she was working at Microsoft. Wouldn't this mean their relationship shouldn't have happened/is wrong?
A better way would have been to make it explicit. i.e. "Hey X, now that we are done with business, I was wondering if you would like to meet over coffee sometime later. This will certainly not affect our professional relationship either way, but I think you're an interesting person and would like to know you better."
Some relationships though are just really dicey, like the one above. Its like having a relationship with your direct manager, since the manager is evaluating you professionally, and that's why its generally considered a bad idea.
Who knows exactly what it was like for Bill ans Melinda. It isn't difficult to pick up on social cues of someone illustrating a mutual interest in you before you outwardly hit on them. It's also not difficult to respectfully ask them if they would like to get a drink, in a personal context. There's no evidence McClure did either.