My love said, “Don’t worry, today is a safe day,” so there’s no need for protection. But I can’t really trust that; after all, the calendar method isn’t always reliable. How can I explain this without hurting their feelings, or should I just say a direct no? What’s the right approach in a situation like this?
So the calendar method is definitely not a reliable method. The cycle doesn’t always work like clockwork, and stress can even affect it. Explain this to her calmly and be clear. If you don’t want to, you can say no; if she gets upset, your health is more important 
But isn’t it already unsafe for a few days right after the period? They taught us in biology at school that fertilization happens later.
@mahrem_saf what kind of information is this? Come on, Güzin Abla. My ex said to me exactly, ‘they all happen on the 14th day anyway,’ but let me put it this way: I found out I was pregnant when I was 2 months along. It’s ridiculous, really. Don’t trust the calendar or anything.
@mahrem_saf are you serious? Did you think the risk is zero right after your period ends? That’s definitely wrong. And then you say this, and later it’s like ‘what’s this surprise?’ 
@mahrem_saf actually, my cycle isn’t very regular. So sometimes there can be a delay of 4-5 days. That’s why I can’t rely on it. But to avoid hurting feelings, to be honest, I’m hesitant to say no directly.
@soran, why would you put yourself at risk so that he doesn’t break? He should be the first to think about it; in such an important matter, he needs to consider his health anyway. So he needs to take responsibility too 
If you want to explain it without breaking the news, you could say: ‘My cycle isn’t regular, and I don’t want to take risks by relying on a calendar.’ If they’re serious, they’ll understand you. For example, you can go to the pharmacy together and get information about different methods of protection.
@skeptik_hesapci is right, but let me add: In these kinds of matters, women generally make sacrifices, and then their value is not recognized. Make your boundaries clear, so that it doesn’t end up like it’s your fault. We’re all watching the same movie.