I would venture a guess that most people on this thread have been following Benedict's career for at least two years. During that time, we have also seen plenty of photos and interviews where he becomes more of himself than what he projects on screen. And a lot of that bubbly goofiness seems to have disappeared since he announced their engagement (though it seems to be coming back in spurts), and well be attributed to his crazy work schedule.
Yes, you have a point that too many people are far too invested in his private life. It's not like we are his friends or relatives, we are fans - no more, no less. That said, I do think it's fair to say that we like him as a human being and want him to be happy. I think many of us have our own idea of what happiness is, and currently we are also viewing his engagement and impending fatherhood through our own lenses. A lot of the BCAS members are either married or in partnerships, some have children, and we probably knew our partners for a while before we made that jump (marriage, cohabitation, children). In this case, we are looking at it through a small lens. We only first heard about SH this past summer - and now he's engaged and going to be a father. If the lead-up to all this had been at least a year in coming, I don't think anyone would be upset. It could come out later that they have been on-again-off-again for years, just that they have been very discreet about it; we will probably never know. But yes, it really is none of our business what he does, with whom and when.
But you also asked why SH would marry Benedict. He is far more famous than she; if she wanted to get ahead with her career, being linked to a shooting star wouldn't be the worst way to do it. They will probably have professional help for the child after it's been born, so she can get back to her career quickly - making new contacts and networking while pregnant would be natural step. I personally don't believe they're doing the baby/wedding thing to improve his chances of winning an award; I don't think he is that superficial. My gut tells me he feels he is doing the gentlemanly thing after an accidental pregnancy. I don't think she's doing it solely for her career, but being linked that closely to Benedict isn't going to harm her either. IF they get married and IF they get divorced, I think she would be the one to come out ahead of where they started.
Sorry if the essay format offends, but you asked questions, so I figured I would answer.