A: During intercourse, the sliding motion of the penis causes pressure on the anus, which lies next to the vaginal wall. The back and forth action can easily cause gas pockets that evacuate. Sometimes, gas becomes trapped in the vagina (very normal, by the way). So when you climax, the muscles around your genitals are relaxed and may also allow a little gas to escape.
Ask your doctor to check to see if there may be a physical cause which can be addressed. If this happens frequently, you may want to take an over-the-counter anti-gas medication, or better yet, have a bowel movement prior to having sex.
For a more permanent fix to this dilemma, change your diet to eliminate carbonated drinks and foods that are hard to digest. These tend to cause excessive gas, bloating and burping.