I don't think Susan was saying anything about your intelligence level, or to say that we all haven't made mistakes, but she just wanted to make sure you knew how it is possible to become pregnant, so that you can protect yourself!
It most likely was not sperm that leaked through to your underwear (through the crotch of your denim jeans), but was most likely YOUR vaginal secretions that you noticed that made your underwear wet (and, possibly the inside of your pants). The outside of your pants would probably be semen.
We didn't understand the necessity to wash your clothes, as sperm can not live outside the body once it has dried, so you can not get pregnant from sperm that is left on clothing. The only way you can get pregnant is if sperm comes into contact with your vagina, and that is not through clothing.
There is some information about what you actually engaged in, called ""outercourse", which is actually a very SMART thing to do if you are deciding to become sexually active. (Sexually active does not mean "having sex", but means engaging in sexual activities that can include kissing, touching, genital-to-genital contact, etc...all with different degrees of risk for pregnancy). So...keep on practicing "safer sex" methods as you are comfortable, and don't do anything you will later regret or worry about!
Ask us if you have any more questions!