My Answer
Given you only care to express that you dislike it, you should ignore or make a gesture/noise dismissing it. He may continue to make these comments a few times more, but it'll naturally stop over time once you stop feeding into it.
Some ways you could dismiss it:
- Ignoring the comment completely or immediately changing the subject
- Making a "Hmm" sound acknowledging the comment but not responding to it
- As CrazyCucumber said, suggesting that thinking about your coworkers that way is weird ("Eh, I feel weird thinking about my coworkers sexually")*
* I'm not a huge fan of this one in all situations. If the person is very macho/overbearing, this might come off as looking down on them while a more logical/mindful person won't.
The reason I suggest dismissing it instead of directly addressing it is because addressing it will definitely put them down even if it's a subconscious feeling to them. It also has the potential to have him label you as a spoilsport.
Why?
Let's take a journey into the mind of someone who makes these comments. I wouldn't necessary call this person a sexist, but rather someone someone who is rationalizing their sexist behavior. This person views objectifying women in this way as normal human male behavior. This person does not share the same fundamental views as you and does not think the same way as you.
When you tell someone what they have internalized as normal is bad ("Not cool.", "You can't say that.", etc.), you're challenging their concept of normal and of themselves. You're challenging something that is core to them in some way. This type of person does not take kindly to being told some core aspect of themselves is "bad". They get defensive, shut down, and stop listening. Think about how many times this has happened during a political, ethical, or religious discussion; it's the same concept.
Instead of being issued a direct challenge, this person needs to feel as if they are challenged by their own perceptions. They need to look at a situation and put the pieces together themselves to figure out what they did wrong. This person needs that moment of "Oh crud, what did I do wrong?" that inevitably leads them to the thought "Maybe commenting on the secretary isn't cool." They need to come to that conclusion themselves and won't accept someone else coming to that conclusion for them.