When most people talk about 'having sex' they are usually referring to sexual intercourse (or penetrative sex). It is also possible to be 'sexual' without actually having sex. Things like kissing, touching, rubbing and stroking are all things that can make you feel good.
Physical reasons: Pleasure, stress relief, exercise, sexual curiosity, or attraction to a person. Goal-based reasons: To make a baby, improve social status (for example, to become popular), or seek revenge. Emotional reasons: Love, commitment, or gratitude.
If we talk of the process of sex, it is simply about people enjoying and indulging in an intimate session with their partner, which can include foreplay, cuddling sessions, kissing, hugging and penetration. For different kinds of sexual orientations, sex is different.
Sexual excitement reaches its peak. You feel a series of intense muscle contractions as your body releases the tension. The muscles of the vagina and the uterus contract. Muscles at the base of the penis tighten and release, releasing semen in an ejaculation.