Your girlfriend and roommate bonded when you went on your weekend fishing trip. You were reeling in an 8 inch trout, while she was taking 8 inches of cock.
Your girlfriend needed a ride but didn't have money to pitch in for gas. Good thing she's got a valuable set of skills and assets that can be used to barter.
You were too tired to go to your friends' party, but your girlfriend wasn't! Here's what she looked like when she got there at 9 pm, and then at 2:30 am, multiple drinks later
When you suspected your girlfriend was cheating, you set up a hidden alarm clock camera. It looks like they've been having risky unprotected sex for awhile :(