There are usually a few weeks of great weather in San Francisco in April and I try to head to the beach during that time...before it cools down again in the summer. Marshall's Beach at the base of the Golden Gate Bridge is an amazing place to strut your stuff. It's the gay "nude" beach, but most of the guys are in some sort of small gear.
Also, you can walk around The Castro in nothing more than a sock on your junk, so showing off in a thong is OK if you are that daring.
If you want to show off in a group of guys, the "gay beach" at Dolores Park is gear friendly. It's the highest part of the park.
If you want more of a bar scene, try the bars in SOMA. They tend to be edgier and more open about that. Of course, check the Internet for underwear parties in SF.