Having worked around the clock at Gold's Gym on Castro, it is a pretty safe bet you are going to get a better work out 5am-7am, 12pm-2pm, and/or 10pm-12am.
The gym staff is pretty good, people are not as "professional" as some would like, but it is truely a community gym.
Yes, one might feel like a pig at the meat market during certain times of the day, but sometimes it is good for the ego. I do have to caution. During those times, testosterone is flowing and people are a little less forgiving in letting others "work in".
As for finding someone to workout with, please start with a trainer. I remember seeing a lot of men taking the fast track to injury. Men might flock to you to be your workout partner, but please get their credentials first.
The SOMA Gold's is also a nice environment, bigger, more industerial, but nice. I like it because there is more equipment, and different classes.
Most of the 24 Hour Fitnesses I have been into have rubbed me wrong. Taking a tour showed the dirt, and grime. I do however like the fact, there is a pool in some of their facilities.
Club One is really nice too. A bit more expensive, but I found it more of a countryclub. Kind of like LA Fitness.
Crunch at Embarcadero is nice and clean, but one has to wait for a machine because most business people go to the downtown gyms after work.
A note about cruising: it happens at every gym to some degree. To a greater extent in the lockerrooms. It will happen, but hey at least you are catching someone's eye. Even if that someone is not your type.
Whatever your choice, I hope you have fun, and tour the gyms first. Try to get a weeks free membership to test out the place. Also, if it is close to the end of the month, and the offer seems too good to be true, there will be another promotion next month.
If you do happen to stop by Gold's on Castro, the two people to talk to are Matt and Jaye for membership.