Sexuality or sexual orientation is not the issue. The commodification of women is the issue. When you list "Women!" as a feature of your event along with "Booze", "Gym Access", and "Food Truck" wares [1], you are treating them as commodities/paid for services like, well, booze, gym access, and food trucks wares. The unfortunate juxtaposition of "massages" and "women" in their list of perks makes the whole thing more unseemly (moreso than they probably intended).
Imagine you are a new female developer in Boston, and you're considering coming to this API jam, and you see that they are promising "women" to people who sign up to come.* Well, now you have to determine a) whether the organizers will see/treat you as a professional and not just a treat to dangle in front of lonely male nerds and b) whether others attendees have certain "expectations" about your reason for coming, based on what they were promised in the event promotion. This is bad. I won't speculate as to whether or not that would prevent anyone from coming, the fact that these considerations would even exist are unacceptable.
[1] http://i.imgur.com/HNdWg.jpg *I know this opens my argument up to "well I'm a woman and I don't see it that way" but please note that the assertion that follows doesn't claim any particular person would react this way but that such a reaction is reasonable/likely.