Ok, latest theory among, mainly male ;-), but also female medics is that swallowing is positive, and women should do it with their partners, if they can do so.
The medics have a sensible reason though, and its not just being a bunch of perverts.
There's a strange, very serious disease that can be fatal to mother and/or baby. Late on in pregnancy (typically) the mothers blood pressure can go sky-high; that's called preeclampsia. If it goes high enough, the mother and/or baby may die; usually the doctors will abort or induce delivery if the pressure goes high enough. It's believed that placenta isn't able to provide enough nutrients, and when the baby gets big enough to need to, the baby generates a chemical to raise the mothers blood pressure higher to force more stuff through to keep the baby alive until it can deliver safely.
But the interesting thing is that it is most common on first pregnancies, later ones rarely get it. The standard theory is that the first baby causes changes to the blood vessels in the womb that are permanent, and the second one has less problems getting enough.
Interestingly, the latest theory disagrees with this last part and says that perhaps there is an immune response between the mother and the child, and this messes up the placenta. The immune response comes about because the mother hasn't completely learnt the fathers/babies proteins yet- but by the second or third child the keying in is complete, and the problem is less likely.
Ok, so how does the keying-in work? Well- unprotected sex. The more exposure the mother has to the fathers sperm the more chance she has of learning him.
This works through vaginal sex. But it also works at least as well via oral- it's a fact that things taken orally tend to suppress immune response, for example girls that had nickel braces on their teeth rarely are allergic to nickel jewellery, and even more commonly food rarely creates an immune response, inspite of the millions of odd chemicals there must be in it; and it is very reasonable to suppose this applies to sperm too.
So, oral sex good- swallowing better.
That theory also explains why a woman swallowing would be considered good from a man's point of view- it signals that the woman likes him a lot and is willing and preparing to have children with him. He won't necessarily know why
he likes it, but he knows he does like it; if it increases the chances of having healthy babies, then evolution will want him to like it.
Ok evidence? There is some, but further research is underway. There is some evidence that mothers that change partners become more susceptible to preeclampsia for example, and it's looking like some men tend to trigger it (they lack certain compounds in their sperm to trigger the keying in process).