you don't really have to. just act normal.
Pretty much. For me somebody being gay mostly is a "as long as you don't trouble me" thing. I mean, during my university time, one of our gaming group at some time, when we were out for food and some beer was like "i have something important to tell you". When he told us what was up, the only question of relevance was like "so, you have a crush on any of us?" As the answer was a no, we told him that it changes nothing and moved on. And indeed it never made a difference.
The only really odd experience i had was during a rock festival, where some guy came to us, started spending beer. We thought he tried to get to one of the girls we had along, no big deal for most of the time. (Would not have been the first time and the girls were fine with it. ) When it turned out that actually i was the target, things got a bit strange for a moment, but it also was clarified quickly. (And hey, for the rest of the festival we were teasing the girls, that they'd have to shape up, the guys out there obviously found me more attractive than them.

)
Stories aside, i also don't think that i'd buy or use the paintjob suggested here. It's not about forcing people to use it, but to have the option to get it. Which might actually be the most logical reason for FD not to do it: evaluating how much effort it would be to make it vs. potential income.
Adding it would be mostly a PR move, when just looking at it from the sales vs. effort point of view, it probably is not worth it. That being said, i'm still all for having the option in game for those who would like to fly that flag around.