Your body/immune system have already been exposed to your partners germs & cells, in fact your body has processes in place to protect your baby who shares some of your partners genetic material. Your partner, whether using his hands or his "wanker" is the least of your infection worries.
In the United States, our hospitals are notorious for causing infections. Though they try to keep things sterile, your chances of getting antibiotic resistant bacteria is higher in the hospital than almost anywhere else! So I wouldn't be completely assured that an internal exam whether with a gloved hand or a internal u/s is completely germ free, and the germs you are likely to get in the hospital are likely to be the bad ones. (I came home from the hospital after my son was born only to go back to be treated for cellulitis--as staph infection I picked up at the hospital. I'm told its probably from my episiotomy incision---something wasn't as sterile as it was supposed to be. And my grandpa almost died from abx resistant staph he got in the hospital after a hip surgery. I know 3 women whose C-section incisions wouldn't heal because of abx resistant staph--from the hospital OR. It's very common to get an infection from the hospital.)
Most internal exams take place in labor--most likely you are not having sex in labor after your water has been broken. But women get several internal exams in labor to check their progress. This increases the risk of infection especially once your water is broken.
And yes, a woman who has had a baby will have a slightly dilated cervix. There's a mucous plug in place which protects your bag of waters from breaking & infection. But many women lose their plug 3-4 weeks before baby is born--during the time in pregnancy where they get internal exams to check dilation at their regular OB appointments.
