Is h. pylori bacteria really contagious? is it contagious say through kissing or sexual activities? can it be cured with only strict diet?

Kissing yes!: Mainly any form of sharing saliva. Also you need to treat this with the physician, who can give you the appropriate medications and antibiotics for a full course. Then this can be confirmed by a breath test that your negative for h. Pylori. Not just diet alone works.

No: The theory is that it lives in very low numbers in the colon, then due to change in that environment, it migrates upstream to the stomach where it causes inflammation and pain. Hp can tolerate a highly acidic environment, where other common colonic bacteria cannot., hence the ability to migrate into a more acid environment. See your doctor for appropriate antibiotics.