There are mainly 4 ways through which a normal young person can develop ED :-

1) You have a low bodyfat % already. You may look like you are fat but you are simply lacking muscle mass in right places.

2) You are cutting calories very drastically. Which i believe you are doing.

3) The food you are eating lacks enough macro/micro nutrients to keep your body in working shape. Which i also believe that you are doing.

4) You have high cholesterol levels & your blood vessels are being blocked due to this. The penile & vaginal blood vessels are like the smallest blood vessels in your body. Hence why they develop blockages quite easily.

In case 1, 2 & 3 i would recommend you to up your calories, start eating healthier foods & start working out.

In case 4, i would suggest you to visit a doctor & get your ldl cholesterol levels checked. A good ldl cholesterol level is around 50 mg/dl.