I have the same problem. I swore I must be allergic to his semen! I would even get a rash sometimes if he came on me. Two possibilites, Bacterial infection, like has been mentioned, or mans PH of his ejaculate is off which can indicate problems, both of you get checked. It is very uncomfortable for the girl and better safe than sorry.
Trish
Are you possibly in the military or been overseas? A lot of women have been complaining of the same thing and their husbands are in the military and have been overseas.
Men and women can pass bacterial infections and yeast infections back and forth, so even if you say "I have no unusual symptoms of anything," you should get checked along with her.
she could have a bacterial infection....often times there are no symptoms associated with this type of infection...she will have to get some cream or pills if that is what it is.