I will be honest. Beach weather in the bay area is a bit crazy. One moment it can be sunny, and the next moment...well, Armageddon has occurred and it looks like a scene from The Fog (a horror flick). And this can occur anytime during the year! But the winter gives something that the summer cannot...really cool skies, less crowded beaches, and plenty of opportunities to snuggle. And if you have kids, the best time for sunset beach photos will always winter, spring, and fall. Why, you may ask? The sun sets at a reasonable hour (i.e., before 8pm).
Oh yeah, and the beaches will likely be chilly 90% of year, so bring a blanket to cuddle under.