In any versus or training mode, pick Remedy or Frallan.
Have a second player or CPU be the other one.

(For extra immersion, pick a Princess Remedy stage as well.)

Get right up on that other character's face and grab them.

Press the Taunt button to give your lover a big ol' smooch.

(This can go both ways, but only between Remedy and Frallan.)

Congratulations! You have achieved the big gay!

Now go online and smooch any Remmies/Breadrolls you see to assert dominance as the ultimate gay!
Bonus points if you do this in tournament while on stream.