Can seamen be used as a spermyside????

a mate told me if i dont wanna get my g/f pregnent i shud rub sum spunk on a condom b4 i use it, then it counter reacts to the spurm if it gets thru and kills it. ive been doin this for 3 weeks and my grl hasnt got her period waz my mate just being a dik? or is she prob just late or sumthing??