Happily married men don’t need a relationship on the side. Good people are not liars and untrustworthy.
Don’t do this. You are a better person than that. You deserve more than to be someone’s second (assuming you are not a third or fourth and that he has other women on the side you don’t know about).
San Francisco is an awful place to date sure but you need to keep looking because there are good men out there. I suggest divorcees or widowers. They often have much more maturity, are much more open and forthcoming, and they know exactly what they want and what they don’t want. They are also not being deceitful to someone who should be important in their life.