When my boyfriend & i have sex it burns when he ejaculates, & its been happening for a while like for around 4yrs & i smell a foul odor after two days.

See answer: Burning with ejaculation is most often related to a non-bacterial inflammation of the prostate, often because of either too frequent or more commonly less frequent ejaculation, and tends to be relatively short-lived and self-limiting. If this has truly been a persistent problem as you mention, would recommend evaluation by a urologist to rule out other potentially significant less common causes.