This is your bias. It's a picture from Halloween or a costume party. The woman is dressed as a nurse. The guys are dressed as geeks. Yes, she's supposed to be sexy (she chose to dress as a "sexy" nurse). I'm not sure how that's objectifying. She's an object because guys find her image attractive? What does that make the guys in the image? Not even objects?
If Tom's job were to try to correct the male/female imbalance of Google+, I would agree that the image is inappropriate. But that's not his job. It's neither his job nor his responsibility to curate his posts to try to never offend the easily-offended.
As for why women are reluctant to join Google+, I think it's got very little to do with the fact that it's "male-dominated" and a lot to do with the fact that most of them simply don't care about it. Their friends aren't there. The people who are there are 99% geeks. As it is now, there's simply little value to Google+ fr them.