I used various hormon birth control (normal pill, mini-pill, plaster, ...): On one side they worked great. I got so tired from them, that I slept half of the day, the other half of the day I was working, so there was no sex at all = 100% safety. ^^
Beside that it made me tired at hell, specially the oral pill is a bit buggy in my eyes. If you have any illnesses, fast-poop, vomiting, whatever ... the reliability of the oral pill is already gone and you need to relie on something else. Certain medication influence with it... With the mini-pill, I had the additional prob, that you need to take that on time every day, and if you forget, reliability was as well gone. So we hardly had any sex, and additional needed another kind of birth control half of the time. -.-
IF you have your period in an reliable cycle, those body-heat/hormon-level computers of the pharmacy might be an option and are rather reliable. With rather I mean, that I would not use it, until you and your partner are willing to take that little risc of an pregnancy. My partner and I have been together for 14 years, and we knew that we wanted kids, once house renewing is done...so god damn, if an "accident" happened, it would have been ok as well, kid simply would have needed to wait for its own kids room.
Personally I was very happy with an copper IUD. They are a bit outdated, so sadly about 10% of woman have troubles with it, and need to remove it again, but if it works for you: They work without hormones, causing no side effect from that. Once its in, you cant forget about anything of it, so no need to watch for diseases, forgetting something, ... As well that, once removed, you are instantly fertile again. Most of my friends taking hormon birth control, had to wait half a year, until their hormon levels were back to normal and they got pregnant. With copper UID you are simply supposed to wait for a period, so that eventual scratches from the removing heal, and thats it.
Diaphragma shall be safe as well, the one thing that kept me from trying it, was that you need to adjust in the middle of intimacy with some kind of item. I prefer simply cuddling with my partner, and if lust comes then its ok, and if not its not. But I really dislike that you cant simply be spontanous, in my oppinion.