If you should ever truly want that then look for the apparent "Gay Couple" and begin with some small talk to break the ice. If they are friendly then maybe suggest something about being curious about the lifestyle. Gain their trust and start a friendship...
I like how you've described it as "lightly". Well we actually got together a number of times and did most everything. Kissed, heavy fellatio, anal coitus and great conversations. There wasn't any double anal or ATM though.
We could be out and around and no one might think that we were three gay guys.
We could be out and around and no one might think that we were three gay guys.