Can a female fail a drug test if her boyfriends dirty sperm mixes with her urine?

No you can't because your pee comes out or your urethra. I assume your boyfriend uses drugs but I would say that if you take the drug test a day after having sex then you would be fine none the less. And pee is the only substance that actually has the thc metabolites in it. :)

Can a man's sperm cause a females drug test to come back positive and up to how long after they have sex?

I discharged once and lost erection what is the problem

No! But I don't know anyone who really likes to do that anyway. Bleck!