missing testicles

No. A man needs both his testicles for normal sexual functionating as the hormone testosterone must be circulating. At least one testicle must be present. As the testicles produce the sex hormones, these are the ones that give you the urge for erection, so if you have no testicles, you can’t be having an erection and the sex drive would be also low, or wouldn’t be having it at all.
There is also a case of having both testicles removed due to a prostate cancer, but this is pretty rare.
I know these few facts about what you have asked and I hope they helped. I asked a friend of mine about this as his brother had a surgery of his testicles and he has been through all this a year ago.
What about you, why curiosity?

My boyfriend has had surgery in his testicles because of a post vasectomy infection. He says the doctors had to permanently "burn" some ducts in both his testicles and can no longer produce sperm. However, he has normal sex drive and can have an erection and ejaculate. Can someone help me understand what type of surgery this is, and, if his testicals have been permanently damaged, is he really infertile for ever? How come he can still have an erection?

Once the testicles are producing the hormone testosterone his body will produce semen hence the sex drive..