The response was familiarly glib, "Because all men are violent rapists and must be punished." We laughed and moved on to the next show. But later, I wondered about it some more. Why exactly was that segment included? It served no story function, did not advance the plot, and was not used in characterization. I came up with a few reasons, some better than others.
Diversity. Because.... DIVERSITY! This could just be activist placard-waving. A poor reason, but not uncommon.
The Gay/Feminist/Liberal Conspiracy. Much as I find that thought entertaining, we can't get organized political parties to agree on a platform, let alone unorganized individuals without centralized communication.
Tokenism. Lots of popular media includes a member of a demographic for no reason other than they want to appeal to the demographic. This commonly services no part of the story, and was my first thought.
After I had time to consider, I came up with a much better, more plausible reason.
Social Trespass. The violation of social norms always evokes an emotional response, commonly titillation or revulsion. Arrow usually does this through displays of intimacy, showing us bare flesh or girl-on-girl scenes that evoke a strong response from male viewers. There were not intimate scenes in that episode, but the discussion of homosexuality serves the same purpose. The predominantly heterosexual male audience of Arrow will react, providing a release of tension before the action stars. That is, after all, what happened. We laughed, we joked, we shrugged off the uncomfortable topic and enjoyed the tension that ratcheted up shortly after.
It's a pacing device, and it works well. But why use this topic when another would have served the same purpose? The characters of Arrow are aggressively male archetypes, even the women express themselves through violence and relational dominance, serving masculine roles instead of feminine. Battlefield sex is as much a part of that genre as the violence. When no feminine archetypes are available to defuse the story tension (in Arrow, that role has fallen almost exclusively to Felicity, following the death of Moira, Laurel's assumption of the Black Canary title, and Thea's transformation into Speedy) one must be manufactured.
It's a reason that is valid as far as story construction goes, but it's not as much fun as conspiracy theory.