I don't really understand how you can be married to someone and NOT have a conversation about such an obvious thing occurring in the bed...but whatever.
She's ejaculating. Women can do it too, and it's just like what you describe. It's not urine, but it does exit through the urethra. Just google "female ejaculation" and you'll get tons of info. Some guys think it's the hottest thing ever. Some hate it. If she's not able to control it then you just need to find ways to deal with it. I think most couples who experience this invest in a plastic sheet for the bed in order to avoid it soaking into the mattress. If you do a google search for "incontinence sheet" it seems to bring up some viable results. Maybe you can put that down overtop your normal sheet so when you're finished with sex you only have to remove that for cleaning and don't have to do the laundry as much.