I think that it's probably something a bit more clever.
You see, most openly gay people will already know the locations of the sort of resource advertised. If they want porn or dating/sex sites (I'm not entirely clear on what the ad is advertising; as I'm gay Facebook hasn't shown it to me), they will already know where to get them.
Assuming that Facebook's ads pay per click rather than per view, though (I will confess to being a little unclear on this point, and couldn't be bothered to go and look), it actually makes a lot of sense to show this stuff to people who say that they're straight in their profiles. Why? Because those of them who are actually closeted gay or bisexual won't have been exposed to such material, and are far more likely to click, and to use the site, whatever it may be.
So, really, it's probably less a case of broken advertising and more a case of clever advertiser.