Robert Pattinson 'only hooking up' with Kristen Stewart, why? 'Twilight' lovers experiment with 'open relationship' as romantic birthday reunion approaches

Apparently, Robert Pattinson only has eyes for Kristen Stewart (although their relationship isn't exclusive).

The British heartthrob recently flew-out to LA after nonstop filming for the new movie "Life" in Toronto. And what perfect timing, K-Stew's 24th birthday is on April 9th!

According to HollywoodLife.com, R-Patz and several "Life" co-stars were spotted hanging out at Hollywood's popular nightclub, The Den. After an evening full of socializing, Pattinson hopped in his car and headed back to the legendary Chateau Marmont. A "mystery" woman was spotted driving the 27-year-old actor and his bodyguard back to the hotel, sparking rumors that Pattinson was hooking-up with another girl behind Stewart's back. But that's not the case at all, according to this alleged insider:

"Rob and Kristen talk all the time and they're still hooking-up, but it's not at all exclusive. I guess you could call it an 'open relationship.' As much as they both love each other, there are no rules in place. It sounds like a crazy way to live but it's so common in Hollywood." 

But here's the catch - Pattinson isn't interested in pursuing other women!

"That's not his style, he's not a huge womanizer the way a lot of guys in Hollywood are," the friend reveals. "Rob would rather get drunk with his friends than pick up girls, and that's what [April 1] was all about. He wasn't there to meet anyone, it was just a fun night with the people he's working with. Who doesn't like to go for drinks after work, right? Rob's no different - once the stress is over he likes to unwind." 

How sweet - the "Twilight" starlet has a good man on her hands!

Do Mstars fans think Robsten will get back together in time for Stewart's birthday festivities on Wednesday?