Earthquakes are common just about anywhere in Greece, most are very minor and cause no problems at all. Santorini has the addition of a live volcano, which can also trigger quakes. Yes, it is possible but unlikely that a major quake will happen when you are there - it is probably more likely that you will be run over by a bus a few miles from home, or even win the lottery.

Personally it does not put me off visiting Greece.
For somewhere romantic, I'd recommend Oia, which is very pretty and is a wonderful place to view the sunset. Fira is lovely and well worth a visit but it rather more touristy and can be expensive. The beaches in resorts like Perissa are black volcanic sand, as long as you don't mind this, they are fine but it wouldn't be my fist choice for a beach holiday.