This is a classic "reductio ad absurdum" straw-man. It's a logical fallacy.
It goes like this: Everything is going to offend someone. Therefore, if we're going to ban anything which offends people, then we need to ban everything. Since we can't ban everything, therefore we must ban nothing.
There are a few non sequiturs there. First, between nothing & everything, there are lots of places to draw the line. We can agree that banning all offense is impractical & undesirable. I think we can also agree that conferences have some call on what content is presented there - even if it's only to ensure that presentations are on-topic - so no editorial guidelines at all is obviously bad also. Between those two extremes, we can draw the line in lots of places: "Don't do anything illegal" covers hate speech, public nudity, smoking, etc. "Don't do anything that standard workplace conventions would forbid" might cover things like drinking alcohol, sexualised content, inappropriate dress or behaviour. The line for free software conferences is probably somewhere between those two.
The thing is: sex is different from a lot of other potentially offending things in our society. Maybe it shouldn't be, but it is. As a father, I watch out for what's on pre-watershed telly sometimes, and there are some pretty violent cartoons and TV shows I don't let my kids watch. Blood & gore bothers me a hell of a lot more than a bit of tit or some bad language might, but in the WASP American culture which has now spread worldwide, sex & bad language are considered worthy of a 15 or 18 age tag on a film, and blood & gore creeps in under PG-13. Go figure.
Plus, we have a situation in our communities that they're seen as unwelcoming to women. Mostly, my guess is that it's because they're predominantly male, and we have more than our fair share of assholes in the free software world, and sometimes they're publicly visible figures.
Add those two together, and it seems clear to me that we need to engage in some positive discrimination to redress the balance and recalibrate the community. That means inviting more female speakers, encouraging female role models, doing outreach specifically to recruit new female developers & community members. If that also means going too far in being paranoid about sexual imagery, then so be it, as far as I'm concerned. The benefit outweighs the cost.
And next time you want to do a presentation where you want to get people's attention or shock, there are lots of potential photos you can use on animal welfare, AIDS awareness, children's charities, landmine awareness, etc. Be a bit more original than tits & ass, s&m. "No sexual imagery" doesn't mean you can't shock & offend.