I couldn't wrap my brain around the idea of using a diaphragm, either, but I did it - it took some getting used to, but it's really not that bad. I've stopped using it, though, because it seemed to make my vaginal yeast infections worse, and I didn't feel comfortable that the contraceptive gel you have to use with it was safe. We've gone back to condoms. My boyfriend says he doesn't mind - it's not his favorite thing, obviously, but "it's better than pushin' a stroller...."
I generally know when I'm ovulating, and we sometimes skip the condom if I'm very close to my period starting. There is also an electronic device, I think it's called "LadyComp," that can predict ovulation - for birth control rather than planning a pregnancy. It's expensive, and I think works largely on body temperature and plugging in your info day after day, and I wasn't sure that my body temp was being affected by a congested liver or sluggish thyroid and might throw off the reading. I tried like heck to get some medial advice from the people manufacturing and retailing the product, but nobody could give me any answers on that question.
Good luck... If you're not afraid of the gel - really give a diaphragm a try. I'm super squeamish - I still can't even use a tampon, but you can't even feel it when it's in, you might surprise yourself. My OBGYN advised against a cap - he said it encouraged infection.