And it doesn't matter if it was meant to be parsed that way. No, the add doesn't outright say "We hire prostitutes to suck your dick at your desk." It doesn't have to say that. But it is being parsed that way by a large number of people. From the looks of this thread, you are in the minority of people who think it is not that way. So what is the difference if it was or was not meant to make a female viewer feel like she is not as important as her male coworkers? The effect is the same. Even if you are right, it is STILL sexist for being so fundamentally inconsiderate.
It's not about being feminist. It's about being humane. There shouldn't even be a term called "feminist", the default should be to treat women as having equal status as men. It shouldn't be a revelation in need of a special name to believe that.
And the fact that you can't see it means you're prone to it as well, and probably towards other groups of people beyond just "women". It's the fallacy of the super-villain: nobody thinks they themselves are the bad guy. Bad behavior is a matter of habit and attitude and prejudice is a matter of lack of respect.
Sexism doesn't end with banning sexually overt behavior in the office. Sexism isn't even about sex any more than racism is about slavery. I would even go so far as to say that a man who talks over a woman in a meeting, but listens intently while another man is talking, is more sexist than a man who flirts with the women in the office. It's about power, and using ones power to give favor to some and deny favor to some others, for no reason other than "they aren't like me".