Mate: Level 8 by Heather Karn

What’s worse than being abducted by aliens? Being told you have to choose one as a mate.

Cammie’s having an average teenage bad day, complete with her car dying on the side of the road, when she’s attacked from behind. As soon as she wakes up, everything Cammie knows about space and extraterrestrials is turned on its head. First, she’s on a real spaceship whose crew proves that aliens aren’t little green people. Second, there are more alien races than she’d considered, and some are at war. If that wasn’t enough, the ship’s captain, Kilani, is adamant he’s not taking her back to Earth for one glaring reason: she’s classified as a Mate, a female able to bear children.
With Kilani’s race dying out, they’ve become desperate to find a solution, but Cammie wants nothing to do with it. All she wants is to return to Earth, that is until Kilani starts showing her what space travel is all about, and that a lonely captain might be just what she’s looking for. However, the question remains for both of them: by the time they reach Kilani’s home world, will he have convinced Cammie to choose him as a Mate, or will she continue to fight to return home?