There are proper places for sexuality, objectification, etc. A professional / workplace environment isn't one of them. It makes people uncomfortable and can contribute to a hostile work environment. The fact is people don't have as much choice over their coworkers as their friends, and they can't easily avoid unwanted interactions as easily as they can in choosing their social circles.

As far as sexism goes when you make assumptions about your audience, you make anyone who doesn't fit those assumptions feel unwelcome. It doesn't mean you should never make assumptions, but always be aware who you are excluding and if you have a good reason for excluding them.