Why does cause such strong contractions?

I know that a man's sperm causes the cervix to soften, and I know that if I have an my uterus contracts, but what I don't get is why after my hubby and I have , and even though I don't climax, I have strong contractions for hours. This has happened every time we've had for the past week and a half. Why aren't they putting me into labor. SN: I am 4cm dilated and 50% effaced at 38plus weeks. Thanks:)