"Minulla on poikaystävää" means "I have (some) boyfriend" in the same way as "I have (some) water" or "I have (some) sand".
"Minulla on poikaystävä" is "I have (a/the) boyfriend"
If it's a countable unit, use the basic form. Boyfriends can be counted, but water and sand can not.
I'm using a german 10 000 mark bill as a bookmark because what else am I gonna do with it? Buy a fraction of a breadcrumb?