Can light sex cause a dvt in thigh and groin to break off?

Depending on what you define as "light" sex it might. I do not know of a validated "degree of sex quesionaire". Discus with your local Dr.

DVT is more common in females than males, especially in child bearing years. Estrogen hormonal therapy and hormonal birth control raises chances the chances of developing a DVT. Any type of adverse movement can cause a clot to dislodge but a normal activity of sex is unlikley.