Can THC be transferred to your sex partner by sperm?
The simple answer is no. If any THC were passed it would be in such a small amount that it would not be detectable.
Another thing to think about here is the possibility of pregnancy and what can be transferred through unprotected sex such as herpes, HIV, Gonorrhea, Syphilis, and Chlamydia among others. If you are going to be sexually active then it is critical that you practice safe sex.
As far as preventing unwanted pregnancies or the transmission of STDs, the only 100% safe method of this is abstinence. But if you choose to have sex, it is generally recommended for teens that they use condoms along with a back-up method of birth control such as the pill or patch. Condoms can help to protect against the transmission of various STDs and condoms, as well as birth control methods, can help to protect against unwanted pregnancies. And it is safe to combine methods such as condoms, birth control pills (or the patch), and spermicides.