Interesting point about the target age group. Where did you find that?
http://bluesky.chicagotribune.com/chi-techweek-chicago-contr...
"Gather with us as we celebrate the launch of our charity initiative with Urban Alliance, the only year long employment program for under resourced high school seniors in Chicago." Excuse any transcription error.
Your question seems incredibly irrelevant. Yes, putting partying people on a party invite seems like the most natural thing to do. The women are clothed, so I'm not sure why you think a shirtless guy is equivalent. But more to the point, I think putting sexy guys having fun on a party invite would make exactly as much sense as this one. But it would have the disadvantage of not showing women that they can show up and have a good time too.
Of everything that you've said on this topic, I agree only with this sentence; neither makes any sense.
P.S. They've unbuttoned their shirts so you can see their muscular pecs. (Note carefully: in my example, they were never shirtless.)