The thing is that there are of course a few people who had sex and went into labour some time afterwards. There are millions more who didn't. Otherwise they would ban sex in pregnancy. There are prostaglandins in semen, same as there are prostaglandins in the chemicals they use to induce but not in the same quantities and strength and in no research ever has sex been shown to induce labour in any way shape or form.
Put it this way, if I said I ate rice and then went to sleep and when I got up the next morning my waters broke!!!, would you believe me if I said rice induced labour? What if I'd eaten a jaffa cake? Or a plum?
It pees me off no end that all these induction techniques, despite having no empirical basis whatsoever, all consist of women having to eat indigestion inducing food, go for mad walks or have uncomfortable sex when they don't want to. Nothing wrong with doing these things if you want to, but telling vulnerable and hopeful women that it will bring on labour is not only erroneous but totally bloody demoralising when it doesn't work in the end.
And to pick up your midwife on a couple of points LuckyOwl, after sex, you shouldn't be encouraging sperm to stay in for significant amounts of time. A wee after sex and letting the sperm out is a way of preventing UTIs (particularly important to avoid when pg) and raspberry leaf tea is not and never has been linked to induction of labour. It's a uterine tonic and pregnant women take it to make their second stage of labour more effective when it finally happens. I have noticed in recent years that it's been lumped together with the induction myths.
And I always say this on these threads, but the logical linking of prostaglandins to labour starting implies that we'd be better off taking the prostaglandins orally. So you need to be on your knees thinking of England or putting it in your tea. Still won't work, but the theory is sounder that way. 