The carrier fluid coagulates briefly, just so it won't run down your leg before the sperm have made their escape. Once the sperm that are going anywhere have gone, the leftover gunk liquefies and runs down your aforementioned leg to form the "wet spot" of lore. If it didn't, you'd be carrying the leftover goo from a lifetime of sexual encounters around with you until you scrubbed it out with a bottle brush, and that's not a pretty picture.
You're fine. However, if your question really does reflect your general state of knowledge about these things, please get a book. Get several. Get a library card. This baby-having business is not simple, and while there is such a thing as too much information, too little information is worse.
Andrea Nemerson has spent the last 14 years as a sex educator and an instructor of sex educators. In her previous life she was a prop designer. And she just gave birth to twins, so she’s one bad mother of a sex adviser. Visit www.altsexcolumn.com to view her previous columns.