Too much? Hard to know how much is "too much". The normal increase in heart rate with sexual arousal is not dangerous. If you already have heart disease or get chest pains with sexual activity get evaluated by your doctor.
Not usually. In a healthy person without heart disease, sex even frequently does contribute to heart disease. If a person has significant heart disease or risk factors for coronary artery disease, any stressful event or strenuous exercise such as sexual activity may precipitate a heart attack.