Hey there! I hope you have a great time in SF!
The Castro is still quite gay, but just FYI many of the businesses are straight owned now. It's still a fun neighborhood, but it's just a tiny section of what the city offers.
Polk Gulch is almost completely straight now (many of the old gay bars are straight, Marina crowd type bars now) and I cant exactly recommend it.
And SOMA has the big dance clubs (The End Up is still open, but not always a totally gay crowd and Mezzanine has gay nights as do a few other clubs like Club Dragon)and other than that you can cover SOMA by hitting the Powerhouse, Eagle, and Hole in the Wall and the sex club Mack (all but the Eagle are within a couple blocks of each other.)
You will however miss out majorly if you don't hit the Mission IMHO.
Most people don't know how gay the Mission has become and you will miss the gay people that are mixed into the Mission's fabric (Almost half the units in my building are gay households for example.)
Basically, the Mission is the "un-Fisherman's Wharf." It's where most of the locals, straight, and gay, go to play. Great restaurants, galleries, bars and shops. I would really suggest at least taking a stroll between 16th and 20th streets on Valencia. They just expanded the sidewalks and put in more trees and some of the best shops are along that section (Paxton Gate and 826 Valencia next door are both pretty amusing and unique window shopping experiences.)
If you want an experience that is a little less whitewashed and where you could use your Spanish, the 24th street corridor from Mission to Potrero is like a slice of Central America.
Hope that helps a bit.
Feel free to email me directly too if you have any specific questions!