I just watched an ABC 20/20 report on online sperm donation. Instead of getting inseminated in a clinic using frozen sperms from a sperm bank, women are now getting "fresh" sperms from donors that they found online. The insemination can be done in two ways, AI (artificial) or natural (NI). With AI, the donor collected the sperms in a cup, which are then transferred to the recipient by herself. With NI, the donor directly inseminates the recipient.
So, if you want to get free sex, register as a free sperm donor in an online registry, and choose NI.