Husband got pre cum on my upper inner thigh while fooling around last night. Already moist environment down there. Should I take plan b?

Plan B may not. be necessary. Pre-ejaculate would need to enter vagina or be deposited at introitus for pregnancy to occur.
Nit Picking sex risk. The doctors here weren't there with you to see if indeed this semen got inside of the vagina. You also failed to tell us where you are in your menstrual cycles and if you are ovulation or not (remember you can only get pregnant when you are ovulating). To get pregnant a female must be ovulating and must have exposure to fresh semen inside of the vagina.