Unless it has been since day one of having sex with him, I would doubt that it has to be due to the sperm. You may have a small tear and/or cut inside your vagina that when he does ejaculate it burns. Some infections could cause this along with some std's too. To exactly pin point this would be hard to do without an exam which your GYN would need to do.
The norm should not be for you to burn, so there is something causing this, so that you can enjoy sex again and just in case it is an infection of some sort, you can get it cleared up so that it doesn't get worse.
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