I was taking finasteride for 3 months with no problems, then I read about the sexual side effect and got them.

If you got sexual side effect after 3 month when you found out that they were a known side effect, then it is likely the power of suggestion, not the medication as sexual side effect, when they occur with finasteride, often happen in the first 2 weeks of so. The incidence of sexual side effects is between 2-4% depending upon which report you read.  Not too common!