A Sotapanna still has kama raga, sex is a part of it. It should be appropriate (not outside the marriage or often changing partners if not married).
Anyway the Sotapanna can see with the Dhamma eye and follows the path, starting with samma ditthi. Sooner or later he/she will see the dangers even in kama raga and also lose interest in more and more sense pleasures … Sakadagami stage follows inevitably. This is at least my experience.