If he has wet semen on his hand when he touches your vagina, there is a possibility that sperm may be deposited which can, with the help of your lubrication, find it's way into your vagina and on their way toward searching for an egg to fertilize.

A lot of things must be near ideal, but it has happened. Sperm are microscopic, and can be present on the tip of the penis long before ejaculation takes place. So, it is taking an unnecessary chance, if you want to avoid becoming pregnant.

If the semen is dry, then the sperm are dead and cannot be brought back to life by moisture.

The safest thing is to use a reliable form of birth control, all the time if you are going to do that kind of activity.