@ Fives
Yeah, I think so. The usual method of establishing reliability is to take 100 couples and see how many get PG over a year when each method is used properly.
The figures I recall are pill = less than 1, condoms ~4, VR ~10.
In typical use tho it's more like 8, 12, and 15. So basically, if you do rhythm properly, it's not massively different from how most people use the pill typically (eg forget to take it, etc).
If I'd had to guess, I'd have said condoms were about 99% reliable and rhythm about 40 to 50. Not so.
The thing with condoms is that you're supposed to put them on the minute the man gets hard, holding it on afterwards and withdrawing immediately.
if however you get him hard with your hand and then you put the condom on ten minutes later, there will be quite a few sperms all over your hands etc already and hence on the outside of the condom before he enters.
Obv used properly they are pretty good but almost nobody uses them properly all the time.