Anxiety and stress. Not stop thinking.

Thinking of sex is nothing abnormal. You don't have to feel bad about that. It is good that you are not in any relationship and don't want either, as that is an external major challenge for you to handle if things don't go as per your expectation. Also with the current thought, you might succumb to the pressure physical desire. Do a self-assessment and list out what are your priorities in life (Educational achievement, career, stability etc.) and focus on being productive. Engage yourself in physical exercises (Gym, swimming, sports of your preference, yoga etc), involve in spirituality, meditation that will keep your mind engaged, spend quality time with your family members. Whenever you get the thought of sex, distract yourself by doing something as mentioned above. You can consider talking to your family and get married and lead an excellent life

Consult a Counsellor who can help you channelise your thoughts