So I found this topic interesting enough to do a little more digging. As tbrock says, there is quite a lot of actual scientific engagement with 'michrochimerism' in the scientific literature. However, here are two important take home messages from what that literature is saying.
First of all, the only testified mechanisms by which a person can get DNA from someone other than themselves is by exchange from mother to child in the womb (maternal transmission through placental blood exchange - this affects male and female children equally), from child to mother in the womb (fetal transmission - this only affects women). Fetal transmission can occur in healthy pregnancies (through placental blood exchange) but doesn't always. It can also occur as a result of miscarriages and abortions, as the foetal cells intermingle in the blood stream and slowly get broken down over time. Another mechanism is organ donation, and blood transfusion. Again, this affects men and women equally, it's just a lot more expensive to test for it in men which is why we don't see more studies on it. Crucially, there is no evidence that coming into contact with sperm alone is a plausible mechanism. It's a hypothesis, sure, but there are good reasons to disprefer it over others. Firstly, because if contact with semen were a way for this to happen, we would expect almost every woman to test positive for signs of microchimerism - and that's simply not the case. However, I will grant you the fact that finding evidence to disprove it as a hypothesis is actually difficult: not because it's plausible, but because to test for it means ruling out any of the other known methods of transfer which is very difficult, at least in women, because if a woman has been having unprotected sex, even whilst on hormonal birth control, there is always a relatively high chance she has been pregnant and not known it (hormonal birth control being not 100% effective, and humans being amongst the most fertile of mammals). This means differentiating foetal transmission, from a putative 'contact with semen' transmission is impossible in women. Now, it *would* be possible to test this theory in men who have unprotected sex with men. However, there are three problems with this:
1) As previously mentioned, testing men for chimerism is hella expensive. It's relatively easy to take a blood sample from a woman, and just look for Y chromosomes that you know couldn't have gotten there in any other way. We can't do that with guys, because we're expecting them to have their own Y chromosomes. So what testing a guy for microchimerism would amount to is taking each individual cell, and asking, 'is this his or someone else's?' over and over and over again. Given that the number of foreign cells/DNA that have been found are teeny tiny, then that's a tough ask. Unless you were lucky enough that the guy had a weird genetic abnormality himself that could be easily screened for, and were certain that none of the guys he'd had sex with had the same thing, then you are shit out of luck on that one.
2) Perhaps more importantly, it assumes that there are no significant differences between anal or oral absorption of semen, and vaginal. Possibly issues there.
3)The final reason it's not been more closely looked at is because it literally wouldn't fucking matter in any case. It's not an important scientific research question, at least not with the knowledge we have at present.
Our best bet is to find women who have had hysterectomies before sexual contact. Rare, but not impossible. However, see the reason above for why no one can actually be bothered to do this.
Reason 3 above in fact leads me nicely to my second important take home message. Having a few cells of foreign DNA in your body does not change **YOUR** DNA. It doesn't necessarily even *do* anything in your body. Certainly not in a way that would alter your facial characteristics. And to the extent it is 'passed on' to your children, it's no more than maternal transmission. Essentially, some extra cells that are NOT your child, hopping along for free. Those cells don't become a part of your child, because the extra DNA in your body is not becoming your DNA. A woman's ovaries are created, with all their genetic material in place, long before she has sexual contact with anyone.
Essentially, what does this kind of intermingling of DNA do (regardless of the source of that DNA)? Well, in a healthy individual whose DNA produces cells that function as they should, probably nothing. If those cells have foreign antigens, the body's immune system attacks them and they are removed, as when a transplant is rejected, or if you gave a blood transfusion of the wrong type. In most cases though, either through careful antigen matching, or the fact that there is genetic relatedness, this doesn't happen. The foreign cells just float around the body, and occasionally they do something they were designed to do. In an unhealthy body, this might be crucial. The transplanted organ replaces the one that doesn't work; the white blood cells replace the cancerous ones, etc. But in a regular body, it's not that extreme. The amount of maternal DNA found in an infant declines over the infants life, as the cells naturally die away. Studies have shown that even the trace DNA leftover comes from natural cell death. Meaning we got that DNA in our body as a child (probably as a functioning blood cell from our mother). When that cell came to the end of it's life, the body kills it off, and at this point, we get free DNA floating about, which is barely detectable, and does absolutely fucking nothing in the body of it's host. (See:
http://www.bloodjournal.org/content/100/8/2845?sso-checked=true)